(* (c) Copyright 2006-2016 Microsoft Corporation and Inria.                  *)
(* Distributed under the terms of CeCILL-B.                                  *)
Require Import mathcomp.ssreflect.ssreflect.
From mathcomp
Require Import ssrfun.

(******************************************************************************)
(* A theory of boolean predicates and operators. A large part of this file is *)
(* concerned with boolean reflection.                                         *)
(* Definitions and notations:                                                 *)
(*               is_true b == the coercion of b : bool to Prop (:= b = true). *)
(*                            This is just input and displayed as `b''.       *)
(*             reflect P b == the reflection inductive predicate, asserting   *)
(*                            that the logical proposition P : prop with the  *)
(*                            formula b : bool. Lemmas asserting reflect P b  *)
(*                            are often referred to as "views".               *)
(*  iffP, appP, sameP, rwP :: lemmas for direct manipulation of reflection    *)
(*                            views: iffP is used to prove reflection from    *)
(*                            logical equivalence, appP to compose views, and *)
(*                            sameP and rwP to perform boolean and setoid     *)
(*                            rewriting.                                      *)
(*                   elimT :: coercion reflect >-> Funclass, which allows the *)
(*                            direct application of `reflect' views to        *)
(*                            boolean assertions.                             *)
(*             decidable P <-> P is effectively decidable (:= {P} + {~ P}.    *)
(*    contra, contraL, ... :: contraposition lemmas.                          *)
(*           altP my_viewP :: natural alternative for reflection; given       *)
(*                            lemma myviewP: reflect my_Prop my_formula,      *)
(*                              have [myP | not_myP] := altP my_viewP.        *)
(*                            generates two subgoals, in which my_formula has *)
(*                            been replaced by true and false, resp., with    *)
(*                            new assumptions myP : my_Prop and               *)
(*                            not_myP: ~~ my_formula.                         *)
(*                            Caveat: my_formula must be an APPLICATION, not  *)
(*                            a variable, constant, let-in, etc. (due to the  *)
(*                            poor behaviour of dependent index matching).    *)
(*        boolP my_formula :: boolean disjunction, equivalent to              *)
(*                            altP (idP my_formula) but circumventing the     *)
(*                            dependent index capture issue; destructing      *)
(*                            boolP my_formula generates two subgoals with    *)
(*                            assumtions my_formula and ~~ myformula. As      *)
(*                            with altP, my_formula must be an application.   *)
(*            \unless C, P <-> we can assume property P when a something that *)
(*                            holds under condition C (such as C itself).     *)
(*                         := forall G : Prop, (C -> G) -> (P -> G) -> G.     *)
(*                            This is just C \/ P or rather its impredicative *)
(*                            encoding, whose usage better fits the above     *)
(*                            description: given a lemma UCP whose conclusion *)
(*                            is \unless C, P we can assume P by writing:     *)
(*                              wlog hP: / P by apply/UCP; (prove C -> goal). *)
(*                           or even apply: UCP id _ => hP if the goal is C.  *)
(*           classically P <-> we can assume P when proving is_true b.        *)
(*                         := forall b : bool, (P -> b) -> b.                 *)
(*                            This is equivalent to ~ (~ P) when P : Prop.    *)
(*             implies P Q == wrapper coinductive type that coerces to P -> Q *)
(*                            and can be used as a P -> Q view unambigously.  *)
(*                            Useful to avoid spurious insertion of <-> views *)
(*                            when Q is a conjunction of foralls, as in Lemma *)
(*                            all_and2 below; conversely, avoids confusion in *)
(*                            apply views for impredicative properties, such  *)
(*                            as \unless C, P. Also supports contrapositives. *)
(*                  a && b == the boolean conjunction of a and b.             *)
(*                  a || b == the boolean disjunction of a and b.             *)
(*                 a ==> b == the boolean implication of b by a.              *)
(*                    ~~ a == the boolean negation of a.                      *)
(*                 a (+) b == the boolean exclusive or (or sum) of a and b.   *)
(*     [ /\ P1 , P2 & P3 ] == multiway logical conjunction, up to 5 terms.    *)
(*     [ \/ P1 , P2 | P3 ] == multiway logical disjunction, up to 4 terms.    *)
(*        [&& a, b, c & d] == iterated, right associative boolean conjunction *)
(*                            with arbitrary arity.                           *)
(*        [|| a, b, c | d] == iterated, right associative boolean disjunction *)
(*                            with arbitrary arity.                           *)
(*      [==> a, b, c => d] == iterated, right associative boolean implication *)
(*                            with arbitrary arity.                           *)
(*              and3P, ... == specific reflection lemmas for iterated         *)
(*                            connectives.                                    *)
(*       andTb, orbAC, ... == systematic names for boolean connective         *)
(*                            properties (see suffix conventions below).      *)
(*              prop_congr == a tactic to move a boolean equality from        *)
(*                            its coerced form in Prop to the equality        *)
(*                            in bool.                                        *)
(*              bool_congr == resolution tactic for blindly weeding out       *)
(*                            like terms from boolean equalities (can fail).  *)
(* This file provides a theory of boolean predicates and relations:           *)
(*                  pred T == the type of bool predicates (:= T -> bool).     *)
(*            simpl_pred T == the type of simplifying bool predicates, using  *)
(*                            the simpl_fun from ssrfun.v.                    *)
(*                   rel T == the type of bool relations.                     *)
(*                         := T -> pred T or T -> T -> bool.                  *)
(*             simpl_rel T == type of simplifying relations.                  *)
(*                predType == the generic predicate interface, supported for  *)
(*                            for lists and sets.                             *)
(*              pred_class == a coercion class for the predType projection to *)
(*                            pred; declaring a coercion to pred_class is an  *)
(*                            alternative way of equipping a type with a      *)
(*                            predType structure, which interoperates better  *)
(*                            with coercion subtyping. This is used, e.g.,    *)
(*                            for finite sets, so that finite groups inherit  *)
(*                            the membership operation by coercing to sets.   *)
(* If P is a predicate the proposition "x satisfies P" can be written         *)
(* applicatively as (P x), or using an explicit connective as (x \in P); in   *)
(* the latter case we say that P is a "collective" predicate. We use A, B     *)
(* rather than P, Q for collective predicates:                                *)
(*                 x \in A == x satisfies the (collective) predicate A.       *)
(*              x \notin A == x doesn't satisfy the (collective) predicate A. *)
(* The pred T type can be used as a generic predicate type for either kind,   *)
(* but the two kinds of predicates should not be confused. When a "generic"   *)
(* pred T value of one type needs to be passed as the other the following     *)
(* conversions should be used explicitly:                                     *)
(*             SimplPred P == a (simplifying) applicative equivalent of P.    *)
(*                   mem A == an applicative equivalent of A:                 *)
(*                            mem A x simplifies to x \in A.                  *)
(* Alternatively one can use the syntax for explicit simplifying predicates   *)
(* and relations (in the following x is bound in E):                          *)
(*            [pred x | E] == simplifying (see ssrfun) predicate x => E.      *)
(*        [pred x : T | E] == predicate x => E, with a cast on the argument.  *)
(*          [pred : T | P] == constant predicate P on type T.                 *)
(*      [pred x | E1 & E2] == [pred x | E1 && E2]; an x : T cast is allowed.  *)
(*           [pred x in A] == [pred x | x in A].                              *)
(*       [pred x in A | E] == [pred x | x in A & E].                          *)
(* [pred x in A | E1 & E2] == [pred x in A | E1 && E2].                       *)
(*           [predU A & B] == union of two collective predicates A and B.     *)
(*           [predI A & B] == intersection of collective predicates A and B.  *)
(*           [predD A & B] == difference of collective predicates A and B.    *)
(*               [predC A] == complement of the collective predicate A.       *)
(*          [preim f of A] == preimage under f of the collective predicate A. *)
(*          predU P Q, ... == union, etc of applicative predicates.           *)
(*                   pred0 == the empty predicate.                            *)
(*                   predT == the total (always true) predicate.              *)
(*                            if T : predArgType, then T coerces to predT.    *)
(*                   {: T} == T cast to predArgType (e.g., {: bool * nat})    *)
(* In the following, x and y are bound in E:                                  *)
(*           [rel x y | E] == simplifying relation x, y => E.                 *)
(*       [rel x y : T | E] == simplifying relation with arguments cast.       *)
(*  [rel x y in A & B | E] == [rel x y | [&& x \in A, y \in B & E]].          *)
(*      [rel x y in A & B] == [rel x y | (x \in A) && (y \in B)].             *)
(*      [rel x y in A | E] == [rel x y in A & A | E].                         *)
(*          [rel x y in A] == [rel x y in A & A].                             *)
(*                relU R S == union of relations R and S.                     *)
(* Explicit values of type pred T (i.e., lamdba terms) should always be used  *)
(* applicatively, while values of collection types implementing the predType  *)
(* interface, such as sequences or sets should always be used as collective   *)
(* predicates. Defined constants and functions of type pred T or simpl_pred T *)
(* as well as the explicit simpl_pred T values described below, can generally *)
(* be used either way. Note however that x \in A will not auto-simplify when  *)
(* A is an explicit simpl_pred T value; the generic simplification rule inE   *)
(* must be used (when A : pred T, the unfold_in rule can be used). Constants  *)
(* of type pred T with an explicit simpl_pred value do not auto-simplify when *)
(* used applicatively, but can still be expanded with inE. This behavior can  *)
(* be controlled as follows:                                                  *)
(*   Let A : collective_pred T := [pred x | ... ].                            *)
(*     The collective_pred T type is just an alias for pred T, but this cast  *)
(*     stops rewrite inE from expanding the definition of A, thus treating A  *)
(*     into an abstract collection (unfold_in or in_collective can be used to *)
(*     expand manually).                                                      *)
(*   Let A : applicative_pred T := [pred x | ...].                            *)
(*     This cast causes inE to turn x \in A into the applicative A x form;    *)
(*     A will then have to unfolded explicitly with the /A rule. This will    *)
(*     also apply to any definition that reduces to A (e.g., Let B := A).     *)
(*   Canonical A_app_pred := ApplicativePred A.                               *)
(*     This declaration, given after definition of A, similarly causes inE to *)
(*     turn x \in A into A x, but in addition allows the app_predE rule to    *)
(*     turn A x back into x \in A; it can be used for any definition of type  *)
(*     pred T, which makes it especially useful for ambivalent predicates     *)
(*     as the relational transitive closure connect, that are used in both    *)
(*     applicative and collective styles.                                     *)
(* Purely for aesthetics, we provide a subtype of collective predicates:      *)
(*   qualifier q T == a pred T pretty-printing wrapper. An A : qualifier q T  *)
(*                    coerces to pred_class and thus behaves as a collective  *)
(*                    predicate, but x \in A and x \notin A are displayed as: *)
(*             x \is A and x \isn't A when q = 0,                             *)
(*         x \is a A and x \isn't a A when q = 1,                             *)
(*       x \is an A and x \isn't an A when q = 2, respectively.               *)
(*   [qualify x | P] := Qualifier 0 (fun x => P), constructor for the above.  *)
(* [qualify x : T | P], [qualify a x | P], [qualify an X | P], etc.           *)
(*                  variants of the above with type constraints and different *)
(*                  values of q.                                              *)
(* We provide an internal interface to support attaching properties (such as  *)
(* being multiplicative) to predicates:                                       *)
(*    pred_key p == phantom type that will serve as a support for properties  *)
(*                  to be attached to p : pred_class; instances should be     *)
(*                  created with Fact/Qed so as to be opaque.                 *)
(* KeyedPred k_p == an instance of the interface structure that attaches      *)
(*                  (k_p : pred_key P) to P; the structure projection is a    *)
(*                  coercion to pred_class.                                   *)
(* KeyedQualifier k_q == an instance of the interface structure that attaches *)
(*                  (k_q : pred_key q) to (q : qualifier n T).                *)
(* DefaultPredKey p == a default value for pred_key p; the vernacular command *)
(*                  Import DefaultKeying attaches this key to all predicates  *)
(*                  that are not explicitly keyed.                            *)
(* Keys can be used to attach properties to predicates, qualifiers and        *)
(* generic nouns in a way that allows them to be used transparently. The key  *)
(* projection of a predicate property structure such as unsignedPred should   *)
(* be a pred_key, not a pred, and corresponding lemmas will have the form     *)
(*    Lemma rpredN R S (oppS : @opprPred R S) (kS : keyed_pred oppS) :        *)
(*       {mono -%R: x / x \in kS}.                                            *)
(* Because x \in kS will be displayed as x \in S (or x \is S, etc), the       *)
(* canonical instance of opprPred will not normally be exposed (it will also  *)
(* be erased by /= simplification). In addition each predicate structure      *)
(* should have a DefaultPredKey Canonical instance that simply issues the     *)
(* property as a proof obligation (which can be caught by the Prop-irrelevant *)
(* feature of the ssreflect plugin).                                          *)
(*   Some properties of predicates and relations:                             *)
(*                  A =i B <-> A and B are extensionally equivalent.          *)
(*         {subset A <= B} <-> A is a (collective) subpredicate of B.         *)
(*             subpred P Q <-> P is an (applicative) subpredicate or Q.       *)
(*              subrel R S <-> R is a subrelation of S.                       *)
(* In the following R is in rel T:                                            *)
(*             reflexive R <-> R is reflexive.                                *)
(*           irreflexive R <-> R is irreflexive.                              *)
(*             symmetric R <-> R (in rel T) is symmetric (equation).          *)
(*         pre_symmetric R <-> R is symmetric (implication).                  *)
(*         antisymmetric R <-> R is antisymmetric.                            *)
(*                 total R <-> R is total.                                    *)
(*            transitive R <-> R is transitive.                               *)
(*       left_transitive R <-> R is a congruence on its left hand side.       *)
(*      right_transitive R <-> R is a congruence on its right hand side.      *)
(*       equivalence_rel R <-> R is an equivalence relation.                  *)
(* Localization of (Prop) predicates; if P1 is convertible to forall x, Qx,   *)
(* P2 to forall x y, Qxy and P3 to forall x y z, Qxyz :                       *)
(*            {for y, P1} <-> Qx{y / x}.                                      *)
(*             {in A, P1} <-> forall x, x \in A -> Qx.                        *)
(*       {in A1 & A2, P2} <-> forall x y, x \in A1 -> y \in A2 -> Qxy.        *)
(*           {in A &, P2} <-> forall x y, x \in A -> y \in A -> Qxy.          *)
(*  {in A1 & A2 & A3, Q3} <-> forall x y z,                                   *)
(*                            x \in A1 -> y \in A2 -> z \in A3 -> Qxyz.       *)
(*     {in A1 & A2 &, Q3} == {in A1 & A2 & A2, Q3}.                           *)
(*      {in A1 && A3, Q3} == {in A1 & A1 & A3, Q3}.                           *)
(*          {in A &&, Q3} == {in A & A & A, Q3}.                              *)
(*    {in A, bijective f} == f has a right inverse in A.                      *)
(*             {on C, P1} == forall x, (f x) \in C -> Qx                      *)
(*                           when P1 is also convertible to Pf f.             *)
(*           {on C &, P2} == forall x y, f x \in C -> f y \in C -> Qxy        *)
(*                           when P2 is also convertible to Pf f.             *)
(*        {on C, P1' & g} == forall x, (f x) \in cd -> Qx                     *)
(*                           when P1' is convertible to Pf f                  *)
(*                           and P1' g is convertible to forall x, Qx.        *)
(*    {on C, bijective f} == f has a right inverse on C.                      *)
(* This file extends the lemma name suffix conventions of ssrfun as follows:  *)
(*   A -- associativity, as in andbA : associative andb.                      *)
(*  AC -- right commutativity.                                                *)
(* ACA -- self-interchange (inner commutativity), e.g.,                       *)
(*        orbACA : (a || b) || (c || d) = (a || c) || (b || d).               *)
(*   b -- a boolean argument, as in andbb : idempotent andb.                  *)
(*   C -- commutativity, as in andbC : commutative andb,                      *)
(*        or predicate complement, as in predC.                               *)
(*  CA -- left commutativity.                                                 *)
(*   D -- predicate difference, as in predD.                                  *)
(*   E -- elimination, as in negbFE : ~~ b = false -> b.                      *)
(*   F or f -- boolean false, as in andbF : b && false = false.               *)
(*   I -- left/right injectivity, as in addbI : right_injective addb,         *)
(*        or predicate intersection, as in predI.                             *)
(*   l -- a left-hand operation, as andb_orl : left_distributive andb orb.    *)
(*   N or n -- boolean negation, as in andbN : a && (~~ a) = false.           *)
(*   P -- a characteristic property, often a reflection lemma, as in          *)
(*        andP : reflect (a /\ b) (a && b).                                   *)
(*   r -- a right-hand operation, as orb_andr : rightt_distributive orb andb. *)
(*   T or t -- boolean truth, as in andbT: right_id true andb.                *)
(*   U -- predicate union, as in predU.                                       *)
(*   W -- weakening, as in in1W : {in D, forall x, P} -> forall x, P.         *)
(******************************************************************************)

Set Implicit Arguments.
Unset Strict Implicit.
Unset Printing Implicit Defensive.

Reserved Notation "~~ b" (at level 35, right associativity).
Reserved Notation "b ==> c" (at level 55, right associativity).
Reserved Notation "b1  (+)  b2" (at level 50, left associativity).
Reserved Notation "x \in A"
  (at level 70, format "'[hv' x '/ '  \in  A ']'", no associativity).
Reserved Notation "x \notin A"
  (at level 70, format "'[hv' x '/ '  \notin  A ']'", no associativity).
Reserved Notation "p1 =i p2"
  (at level 70, format "'[hv' p1 '/ '  =i  p2 ']'", no associativity).

(* We introduce a number of n-ary "list-style" notations that share a common  *)
(* format, namely                                                             *)
(*    [op arg1, arg2, ... last_separator last_arg]                            *)
(* This usually denotes a right-associative applications of op, e.g.,         *)
(*  [&& a, b, c & d] denotes a && (b && (c && d))                             *)
(* The last_separator must be a non-operator token. Here we use &, | or =>;   *)
(* our default is &, but we try to match the intended meaning of op. The      *)
(* separator is a workaround for limitations of the parsing engine; the same  *)
(* limitations mean the separator cannot be omitted even when last_arg can.   *)
(*   The Notation declarations are complicated by the separate treatment for  *)
(* some fixed arities (binary for bool operators, and all arities for Prop    *)
(* operators).                                                                *)
(*   We also use the square brackets in comprehension-style notations         *)
(*    [type var separator expr]                                               *)
(* where "type" is the type of the comprehension (e.g., pred) and "separator" *)
(* is | or => . It is important that in other notations a leading square      *)
(* bracket [ is always followed by an operator symbol or a fixed identifier.  *)

Reserved Notation "[ /\ P1 & P2 ]" (at level 0, only parsing).
Reserved Notation "[ /\ P1 , P2 & P3 ]" (at level 0, format
  "'[hv' [ /\ '['  P1 , '/'  P2 ']' '/ '  &  P3 ] ']'").
Reserved Notation "[ /\ P1 , P2 , P3 & P4 ]" (at level 0, format
  "'[hv' [ /\ '['  P1 , '/'  P2 , '/'  P3 ']' '/ '  &  P4 ] ']'").
Reserved Notation "[ /\ P1 , P2 , P3 , P4 & P5 ]" (at level 0, format
  "'[hv' [ /\ '['  P1 , '/'  P2 , '/'  P3 , '/'  P4 ']' '/ '  &  P5 ] ']'").

Reserved Notation "[ \/ P1 | P2 ]" (at level 0, only parsing).
Reserved Notation "[ \/ P1 , P2 | P3 ]" (at level 0, format
  "'[hv' [ \/ '['  P1 , '/'  P2 ']' '/ '  |  P3 ] ']'").
Reserved Notation "[ \/ P1 , P2 , P3 | P4 ]" (at level 0, format
  "'[hv' [ \/ '['  P1 , '/'  P2 , '/'  P3 ']' '/ '  |  P4 ] ']'").

Reserved Notation "[ && b1 & c ]" (at level 0, only parsing).
Reserved Notation "[ && b1 , b2 , .. , bn & c ]" (at level 0, format
  "'[hv' [ && '['  b1 , '/'  b2 , '/'  .. , '/'  bn ']' '/ '  &  c ] ']'").

Reserved Notation "[ || b1 | c ]" (at level 0, only parsing).
Reserved Notation "[ || b1 , b2 , .. , bn | c ]" (at level 0, format
  "'[hv' [ || '['  b1 , '/'  b2 , '/'  .. , '/'  bn ']' '/ '  |  c ] ']'").

Reserved Notation "[ ==> b1 => c ]" (at level 0, only parsing).
Reserved Notation "[ ==> b1 , b2 , .. , bn => c ]" (at level 0, format
  "'[hv' [ ==> '['  b1 , '/'  b2 , '/'  .. , '/'  bn ']' '/'  =>  c ] ']'").

Reserved Notation "[ 'pred' : T => E ]" (at level 0, format
  "'[hv' [ 'pred' :  T  => '/ '  E ] ']'").
Reserved Notation "[ 'pred' x => E ]" (at level 0, x at level 8, format
  "'[hv' [ 'pred'  x  => '/ '  E ] ']'").
Reserved Notation "[ 'pred' x : T => E ]" (at level 0, x at level 8, format
  "'[hv' [ 'pred'  x  :  T  => '/ '  E ] ']'").

Reserved Notation "[ 'rel' x y => E ]" (at level 0, x, y at level 8, format
  "'[hv' [ 'rel'  x   y  => '/ '  E ] ']'").
Reserved Notation "[ 'rel' x y : T => E ]" (at level 0, x, y at level 8, format
  "'[hv' [ 'rel'  x  y :  T  => '/ '  E ] ']'").

(* Shorter delimiter *)
Delimit Scope bool_scope with B.
Open Scope bool_scope.

(* An alternative to xorb that behaves somewhat better wrt simplification.    *)
Definition addb b := if b then negb else id.

(* Notation for && and || is declared in Init.Datatypes. *)
Notation "~~ b" := (negb b) : bool_scope.
Notation "b ==> c" := (implb b c) : bool_scope.
Notation "b1 (+) b2" := (addb b1 b2) : bool_scope.

(* Constant is_true b := b = true is defined in Init.Datatypes. *)
Coercion is_true : bool >-> Sortclass. (* Prop *)

Lemma prop_congr : forall b b' : bool, b = b' -> b = b' :> Prop.
Proof. by move=> b b' ->. Qed.

Ltac prop_congr := apply: prop_congr.

(* Lemmas for trivial. *)
Lemma is_true_true : true.               Proof. by []. Qed.
Lemma not_false_is_true : ~ false.       Proof. by []. Qed.
Lemma is_true_locked_true : locked true. Proof. by unlock. Qed.
Hint Resolve is_true_true not_false_is_true is_true_locked_true.

(* Shorter names. *)
Definition isT := is_true_true.
Definition notF := not_false_is_true.

(* Negation lemmas. *)

(* We generally take NEGATION as the standard form of a false condition:      *)
(* negative boolean hypotheses should be of the form ~~ b, rather than ~ b or *)
(* b = false, as much as possible.                                            *)

Lemma negbT b : b = false -> ~~ b.          Proof. by case: b. Qed.
Lemma negbTE b : ~~ b -> b = false.         Proof. by case: b. Qed.
Lemma negbF b : (b : bool) -> ~~ b = false. Proof. by case: b. Qed.
Lemma negbFE b : ~~ b = false -> b.         Proof. by case: b. Qed.
Lemma negbK : involutive negb.              Proof. by case. Qed.
Lemma negbNE b : ~~ ~~ b -> b.              Proof. by case: b. Qed.

Lemma negb_inj : injective negb. Proof. exact: can_inj negbK. Qed.
Lemma negbLR b c : b = ~~ c -> ~~ b = c. Proof. exact: canLR negbK. Qed.
Lemma negbRL b c : ~~ b = c -> b = ~~ c. Proof. exact: canRL negbK. Qed.

Lemma contra (c b : bool) : (c -> b) -> ~~ b -> ~~ c.
Proof. by case: b => //; case: c. Qed.
Definition contraNN := contra.

Lemma contraL (c b : bool) : (c -> ~~ b) -> b -> ~~ c.
Proof. by case: b => //; case: c. Qed.
Definition contraTN := contraL.

Lemma contraR (c b : bool) : (~~ c -> b) -> ~~ b -> c.
Proof. by case: b => //; case: c. Qed.
Definition contraNT := contraR.

Lemma contraLR (c b : bool) : (~~ c -> ~~ b) -> b -> c.
Proof. by case: b => //; case: c. Qed.
Definition contraTT := contraLR.

Lemma contraT b : (~~ b -> false) -> b. Proof. by case: b => // ->. Qed.

Lemma wlog_neg b : (~~ b -> b) -> b. Proof. by case: b => // ->. Qed.

Lemma contraFT (c b : bool) : (~~ c -> b) -> b = false -> c.
Proof. by move/contraR=> notb_c /negbT. Qed.

Lemma contraFN (c b : bool) : (c -> b) -> b = false -> ~~ c.
Proof. by move/contra=> notb_notc /negbT. Qed.

Lemma contraTF (c b : bool) : (c -> ~~ b) -> b -> c = false.
Proof. by move/contraL=> b_notc /b_notc/negbTE. Qed.

Lemma contraNF (c b : bool) : (c -> b) -> ~~ b -> c = false.
Proof. by move/contra=> notb_notc /notb_notc/negbTE. Qed.

Lemma contraFF (c b : bool) : (c -> b) -> b = false -> c = false.
Proof. by move/contraFN=> bF_notc /bF_notc/negbTE. Qed.

(* Coercion of sum-style datatypes into bool, which makes it possible *)
(* to use ssr's boolean if rather than Coq's "generic" if.            *)

Coercion isSome T (u : option T) := if u is Some _ then true else false.

Coercion is_inl A B (u : A + B) := if u is inl _ then true else false.

Coercion is_left A B (u : {A} + {B}) := if u is left _ then true else false.

Coercion is_inleft A B (u : A + {B}) := if u is inleft _ then true else false.

Prenex Implicits  isSome is_inl is_left is_inleft.

Definition decidable P := {P} + {~ P}.

(* Lemmas for ifs with large conditions, which allow reasoning about the  *)
(* condition without repeating it inside the proof (the latter IS         *)
(* preferable when the condition is short).                               *)
(* Usage :                                                                *)
(*   if the goal contains (if cond then ...) = ...                        *)
(*     case: ifP => Hcond.                                                *)
(*   generates two subgoal, with the assumption Hcond : cond = true/false *)
(*     Rewrite if_same  eliminates redundant ifs                          *)
(*     Rewrite (fun_if f) moves a function f inside an if                 *)
(*     Rewrite if_arg moves an argument inside a function-valued if       *)

Section BoolIf.

Variables (A B : Type) (x : A) (f : A -> B) (b : bool) (vT vF : A).

Variant if_spec (not_b : Prop) : bool -> A -> Set :=
  | IfSpecTrue  of      b : if_spec not_b true vT
  | IfSpecFalse of  not_b : if_spec not_b false vF.

Lemma ifP : if_spec (b = false) b (if b then vT else vF).
Proof. by case def_b: b; constructor. Qed.

Lemma ifPn : if_spec (~~ b) b (if b then vT else vF).
Proof. by case def_b: b; constructor; rewrite ?def_b. Qed.

Lemma ifT : b -> (if b then vT else vF) = vT. Proof. by move->. Qed.
Lemma ifF : b = false -> (if b then vT else vF) = vF. Proof. by move->. Qed.
Lemma ifN : ~~ b -> (if b then vT else vF) = vF. Proof. by move/negbTE->. Qed.

Lemma if_same : (if b then vT else vT) = vT.
Proof. by case b. Qed.

Lemma if_neg : (if ~~ b then vT else vF) = if b then vF else vT.
Proof. by case b. Qed.

Lemma fun_if : f (if b then vT else vF) = if b then f vT else f vF.
Proof. by case b. Qed.

Lemma if_arg (fT fF : A -> B) :
  (if b then fT else fF) x = if b then fT x else fF x.
Proof. by case b. Qed.

(* Turning a boolean "if" form into an application.                           *)
Definition if_expr := if b then vT else vF.
Lemma ifE : (if b then vT else vF) = if_expr. Proof. by []. Qed.

End BoolIf.

(* The reflection predicate.                                          *)

Inductive reflect (P : Prop) : bool -> Set :=
  | ReflectT  of   P : reflect P true
  | ReflectF of ~ P : reflect P false.

(* Core (internal) reflection lemmas, used for the three kinds of views. *)

Section ReflectCore.

Variables (P Q : Prop) (b c : bool).

Hypothesis Hb : reflect P b.

Lemma introNTF : (if c then ~ P else P) -> ~~ b = c.
Proof. by case c; case Hb. Qed.

Lemma introTF : (if c then P else ~ P) -> b = c.
Proof. by case c; case Hb. Qed.

Lemma elimNTF : ~~ b = c -> if c then ~ P else P.
Proof. by move <-; case Hb. Qed.

Lemma elimTF : b = c -> if c then P else ~ P.
Proof. by move <-; case Hb. Qed.

Lemma equivPif : (Q -> P) -> (P -> Q) -> if b then Q else ~ Q.
Proof. by case Hb; auto. Qed.

Lemma xorPif : Q \/ P -> ~ (Q /\ P) -> if b then ~ Q else Q.
Proof. by case Hb => [? _ H ? | ? H _]; case: H. Qed.

End ReflectCore.

(* Internal negated reflection lemmas *)
Section ReflectNegCore.

Variables (P Q : Prop) (b c : bool).
Hypothesis Hb : reflect P (~~ b).

Lemma introTFn : (if c then ~ P else P) -> b = c.
Proof. by move/(introNTF Hb) <-; case b. Qed.

Lemma elimTFn : b = c -> if c then ~ P else P.
Proof. by move <-; apply: (elimNTF Hb); case b. Qed.

Lemma equivPifn : (Q -> P) -> (P -> Q) -> if b then ~ Q else Q.
Proof. by rewrite -if_neg; apply: equivPif. Qed.

Lemma xorPifn : Q \/ P -> ~ (Q /\ P) -> if b then Q else ~ Q.
Proof. by rewrite -if_neg; apply: xorPif. Qed.

End ReflectNegCore.

(* User-oriented reflection lemmas *)
Section Reflect.

Variables (P Q : Prop) (b b' c : bool).
Hypotheses (Pb : reflect P b) (Pb' : reflect P (~~ b')).

Lemma introT  : P -> b.            Proof. exact: introTF true _. Qed.
Lemma introF  : ~ P -> b = false.  Proof. exact: introTF false _. Qed.
Lemma introN  : ~ P -> ~~ b.       Proof. exact: introNTF true _. Qed.
Lemma introNf : P -> ~~ b = false. Proof. exact: introNTF false _. Qed.
Lemma introTn : ~ P -> b'.         Proof. exact: introTFn true _. Qed.
Lemma introFn : P -> b' = false.   Proof. exact: introTFn false _. Qed.

Lemma elimT  : b -> P.             Proof. exact: elimTF true _. Qed.
Lemma elimF  : b = false -> ~ P.   Proof. exact: elimTF false _. Qed.
Lemma elimN  : ~~ b -> ~P.         Proof. exact: elimNTF true _. Qed.
Lemma elimNf : ~~ b = false -> P.  Proof. exact: elimNTF false _. Qed.
Lemma elimTn : b' -> ~ P.          Proof. exact: elimTFn true _. Qed.
Lemma elimFn : b' = false -> P.    Proof. exact: elimTFn false _. Qed.

Lemma introP : (b -> Q) -> (~~ b -> ~ Q) -> reflect Q b.
Proof. by case b; constructor; auto. Qed.

Lemma iffP : (P -> Q) -> (Q -> P) -> reflect Q b.
Proof. by case: Pb; constructor; auto. Qed.

Lemma equivP : (P <-> Q) -> reflect Q b.
Proof. by case; apply: iffP. Qed.

Lemma sumboolP (decQ : decidable Q) : reflect Q decQ.
Proof. by case: decQ; constructor. Qed.

Lemma appP : reflect Q b -> P -> Q.
Proof. by move=> Qb; move/introT; case: Qb. Qed.

Lemma sameP : reflect P c -> b = c.
Proof. by case; [apply: introT | apply: introF]. Qed.

Lemma decPcases : if b then P else ~ P. Proof. by case Pb. Qed.

Definition decP : decidable P. by case: b decPcases; [left | right]. Defined.

Lemma rwP : P <-> b. Proof. by split; [apply: introT | apply: elimT]. Qed.

Lemma rwP2 : reflect Q b -> (P <-> Q).
Proof. by move=> Qb; split=> ?; [apply: appP | apply: elimT; case: Qb]. Qed.

(*  Predicate family to reflect excluded middle in bool.                      *)
Variant alt_spec : bool -> Type :=
  | AltTrue of     P : alt_spec true
  | AltFalse of ~~ b : alt_spec false.

Lemma altP : alt_spec b.
Proof. by case def_b: b / Pb; constructor; rewrite ?def_b. Qed.

End Reflect.

Hint View for move/ elimTF|3 elimNTF|3 elimTFn|3 introT|2 introTn|2 introN|2.

Hint View for apply/ introTF|3 introNTF|3 introTFn|3 elimT|2 elimTn|2 elimN|2.

Hint View for apply// equivPif|3 xorPif|3 equivPifn|3 xorPifn|3.

(* Allow the direct application of a reflection lemma to a boolean assertion. *)
Coercion elimT : reflect >-> Funclass.

Variant implies P Q := Implies of P -> Q.
Lemma impliesP P Q : implies P Q -> P -> Q. Proof. by case. Qed.
Lemma impliesPn (P Q : Prop) : implies P Q -> ~ Q -> ~ P.
Proof. by case=> iP ? /iP. Qed.
Coercion impliesP : implies >-> Funclass.
Hint View for move/ impliesPn|2 impliesP|2.
Hint View for apply/ impliesPn|2 impliesP|2.

(* Impredicative or, which can emulate a classical not-implies. *)
Definition unless condition property : Prop :=
 forall goal : Prop, (condition -> goal) -> (property -> goal) -> goal.

Notation "\unless C , P" := (unless C P)
  (at level 200, C at level 100,
   format "'[' \unless  C , '/ '  P ']'") : type_scope.

Lemma unlessL C P : implies C (\unless C, P).
Proof. by split=> hC G /(_ hC). Qed.

Lemma unlessR C P : implies P (\unless C, P).
Proof. by split=> hP G _ /(_ hP). Qed.

Lemma unless_sym C P : implies (\unless C, P) (\unless P, C).
Proof. by split; apply; [apply/unlessR | apply/unlessL]. Qed.

Lemma unlessP (C P : Prop) : (\unless C, P) <-> C \/ P.
Proof. by split=> [|[/unlessL | /unlessR]]; apply; [left | right]. Qed.

Lemma bind_unless C P {Q} : implies (\unless C, P) (\unless (\unless C, Q), P).
Proof. by split; apply=> [hC|hP]; [apply/unlessL/unlessL | apply/unlessR]. Qed.

Lemma unless_contra b C : implies (~~ b -> C) (\unless C, b).
Proof. by split; case: b => [_ | hC]; [apply/unlessR | apply/unlessL/hC]. Qed.

(* Classical reasoning becomes directly accessible for any bool subgoal.      *)
(* Note that we cannot use "unless" here for lack of universe polymorphism.   *)
Definition classically P : Prop := forall b : bool, (P -> b) -> b.

Lemma classicP (P : Prop) : classically P <-> ~ ~ P.
Proof.
split=> [cP nP | nnP [] // nP]; last by case nnP; move/nP.
by have: P -> false; [move/nP | move/cP].
Qed.

Lemma classicW P : P -> classically P. Proof. by move=> hP _ ->. Qed.

Lemma classic_bind P Q : (P -> classically Q) -> classically P -> classically Q.
Proof. by move=> iPQ cP b /iPQ-/cP. Qed.

Lemma classic_EM P : classically (decidable P).
Proof.
by case=> // undecP; apply/undecP; right=> notP; apply/notF/undecP; left.
Qed.

Lemma classic_pick T P : classically ({x : T | P x} + (forall x, ~ P x)).
Proof.
case=> // undecP; apply/undecP; right=> x Px.
by apply/notF/undecP; left; exists x.
Qed.

Lemma classic_imply P Q : (P -> classically Q) -> classically (P -> Q).
Proof.
move=> iPQ []// notPQ; apply/notPQ=> /iPQ-cQ.
by case: notF; apply: cQ => hQ; apply: notPQ.
Qed.

(* List notations for wider connectives; the Prop connectives have a fixed    *)
(* width so as to avoid iterated destruction (we go up to width 5 for /\, and *)
(* width 4 for or). The bool connectives have arbitrary widths, but denote    *)
(* expressions that associate to the RIGHT. This is consistent with the right *)
(* associativity of list expressions and thus more convenient in most proofs. *)

Inductive and3 (P1 P2 P3 : Prop) : Prop := And3 of P1 & P2 & P3.

Inductive and4 (P1 P2 P3 P4 : Prop) : Prop := And4 of P1 & P2 & P3 & P4.

Inductive and5 (P1 P2 P3 P4 P5 : Prop) : Prop :=
  And5 of P1 & P2 & P3 & P4 & P5.

Inductive or3 (P1 P2 P3 : Prop) : Prop := Or31 of P1 | Or32 of P2 | Or33 of P3.

Inductive or4 (P1 P2 P3 P4 : Prop) : Prop :=
  Or41 of P1 | Or42 of P2 | Or43 of P3 | Or44 of P4.

Notation "[ /\ P1 & P2 ]" := (and P1 P2) (only parsing) : type_scope.
Notation "[ /\ P1 , P2 & P3 ]" := (and3 P1 P2 P3) : type_scope.
Notation "[ /\ P1 , P2 , P3 & P4 ]" := (and4 P1 P2 P3 P4) : type_scope.
Notation "[ /\ P1 , P2 , P3 , P4 & P5 ]" := (and5 P1 P2 P3 P4 P5) : type_scope.

Notation "[ \/ P1 | P2 ]" := (or P1 P2) (only parsing) : type_scope.
Notation "[ \/ P1 , P2 | P3 ]" := (or3 P1 P2 P3) : type_scope.
Notation "[ \/ P1 , P2 , P3 | P4 ]" := (or4 P1 P2 P3 P4) : type_scope.

Notation "[ && b1 & c ]" := (b1 && c) (only parsing) : bool_scope.
Notation "[ && b1 , b2 , .. , bn & c ]" := (b1 && (b2 && .. (bn && c) .. ))
  : bool_scope.

Notation "[ || b1 | c ]" := (b1 || c) (only parsing) : bool_scope.
Notation "[ || b1 , b2 , .. , bn | c ]" := (b1 || (b2 || .. (bn || c) .. ))
  : bool_scope.

Notation "[ ==> b1 , b2 , .. , bn => c ]" :=
   (b1 ==> (b2 ==> .. (bn ==> c) .. )) : bool_scope.
Notation "[ ==> b1 => c ]" := (b1 ==> c) (only parsing) : bool_scope.

Section AllAnd.

Variables (T : Type) (P1 P2 P3 P4 P5 : T -> Prop).
Local Notation a P := (forall x, P x).

Lemma all_and2 : implies (forall x, [/\ P1 x & P2 x]) [/\ a P1 & a P2].
Proof. by split=> haveP; split=> x; case: (haveP x). Qed.

Lemma all_and3 : implies (forall x, [/\ P1 x, P2 x & P3 x])
                         [/\ a P1, a P2 & a P3].
Proof. by split=> haveP; split=> x; case: (haveP x). Qed.

Lemma all_and4 : implies (forall x, [/\ P1 x, P2 x, P3 x & P4 x])
                         [/\ a P1, a P2, a P3 & a P4].
Proof. by split=> haveP; split=> x; case: (haveP x). Qed.

Lemma all_and5 : implies (forall x, [/\ P1 x, P2 x, P3 x, P4 x & P5 x])
                         [/\ a P1, a P2, a P3, a P4 & a P5].
Proof. by split=> haveP; split=> x; case: (haveP x). Qed.

End AllAnd.

Implicit Arguments all_and2 [[T] [P1] [P2]].
Implicit Arguments all_and3 [[T] [P1] [P2] [P3]].
Implicit Arguments all_and4 [[T] [P1] [P2] [P3] [P4]].
Implicit Arguments all_and5 [[T] [P1] [P2] [P3] [P4] [P5]].

Lemma pair_andP P Q : P /\ Q <-> P * Q. Proof. by split; case. Qed.

Section ReflectConnectives.

Variable b1 b2 b3 b4 b5 : bool.

Lemma idP : reflect b1 b1.
Proof. by case b1; constructor. Qed.

Lemma boolP : alt_spec b1 b1 b1.
Proof. exact: (altP idP). Qed.

Lemma idPn : reflect (~~ b1) (~~ b1).
Proof. by case b1; constructor. Qed.

Lemma negP : reflect (~ b1) (~~ b1).
Proof. by case b1; constructor; auto. Qed.

Lemma negPn : reflect b1 (~~ ~~ b1).
Proof. by case b1; constructor. Qed.

Lemma negPf : reflect (b1 = false) (~~ b1).
Proof. by case b1; constructor. Qed.

Lemma andP : reflect (b1 /\ b2) (b1 && b2).
Proof. by case b1; case b2; constructor=> //; case. Qed.

Lemma and3P : reflect [/\ b1, b2 & b3] [&& b1, b2 & b3].
Proof. by case b1; case b2; case b3; constructor; try by case. Qed.

Lemma and4P : reflect [/\ b1, b2, b3 & b4] [&& b1, b2, b3 & b4].
Proof. by case b1; case b2; case b3; case b4; constructor; try by case. Qed.

Lemma and5P : reflect [/\ b1, b2, b3, b4 & b5] [&& b1, b2, b3, b4 & b5].
Proof.
by case b1; case b2; case b3; case b4; case b5; constructor; try by case.
Qed.

Lemma orP : reflect (b1 \/ b2) (b1 || b2).
Proof. by case b1; case b2; constructor; auto; case. Qed.

Lemma or3P : reflect [\/ b1, b2 | b3] [|| b1, b2 | b3].
Proof.
case b1; first by constructor; constructor 1.
case b2; first by constructor; constructor 2.
case b3; first by constructor; constructor 3.
by constructor; case.
Qed.

Lemma or4P : reflect [\/ b1, b2, b3 | b4] [|| b1, b2, b3 | b4].
Proof.
case b1; first by constructor; constructor 1.
case b2; first by constructor; constructor 2.
case b3; first by constructor; constructor 3.
case b4; first by constructor; constructor 4.
by constructor; case.
Qed.

Lemma nandP : reflect (~~ b1 \/ ~~ b2) (~~ (b1 && b2)).
Proof. by case b1; case b2; constructor; auto; case; auto. Qed.

Lemma norP : reflect (~~ b1 /\ ~~ b2) (~~ (b1 || b2)).
Proof. by case b1; case b2; constructor; auto; case; auto. Qed.

Lemma implyP : reflect (b1 -> b2) (b1 ==> b2).
Proof. by case b1; case b2; constructor; auto. Qed.

End ReflectConnectives.

Implicit Arguments idP [b1].
Implicit Arguments idPn [b1].
Implicit Arguments negP [b1].
Implicit Arguments negPn [b1].
Implicit Arguments negPf [b1].
Implicit Arguments andP [b1 b2].
Implicit Arguments and3P [b1 b2 b3].
Implicit Arguments and4P [b1 b2 b3 b4].
Implicit Arguments and5P [b1 b2 b3 b4 b5].
Implicit Arguments orP [b1 b2].
Implicit Arguments or3P [b1 b2 b3].
Implicit Arguments or4P [b1 b2 b3 b4].
Implicit Arguments nandP [b1 b2].
Implicit Arguments norP [b1 b2].
Implicit Arguments implyP [b1 b2].
Prenex Implicits idP idPn negP negPn negPf.
Prenex Implicits andP and3P and4P and5P orP or3P or4P nandP norP implyP.

(* Shorter, more systematic names for the boolean connectives laws.       *)

Lemma andTb : left_id true andb.       Proof. by []. Qed.
Lemma andFb : left_zero false andb.    Proof. by []. Qed.
Lemma andbT : right_id true andb.      Proof. by case. Qed.
Lemma andbF : right_zero false andb.   Proof. by case. Qed.
Lemma andbb : idempotent andb.         Proof. by case. Qed.
Lemma andbC : commutative andb.        Proof. by do 2!case. Qed.
Lemma andbA : associative andb.        Proof. by do 3!case. Qed.
Lemma andbCA : left_commutative andb.  Proof. by do 3!case. Qed.
Lemma andbAC : right_commutative andb. Proof. by do 3!case. Qed.
Lemma andbACA : interchange andb andb. Proof. by do 4!case. Qed.

Lemma orTb : forall b, true || b.      Proof. by []. Qed.
Lemma orFb : left_id false orb.        Proof. by []. Qed.
Lemma orbT : forall b, b || true.      Proof. by case. Qed.
Lemma orbF : right_id false orb.       Proof. by case. Qed.
Lemma orbb : idempotent orb.           Proof. by case. Qed.
Lemma orbC : commutative orb.          Proof. by do 2!case. Qed.
Lemma orbA : associative orb.          Proof. by do 3!case. Qed.
Lemma orbCA : left_commutative orb.    Proof. by do 3!case. Qed.
Lemma orbAC : right_commutative orb.   Proof. by do 3!case. Qed.
Lemma orbACA : interchange orb orb.    Proof. by do 4!case. Qed.

Lemma andbN b : b && ~~ b = false. Proof. by case: b. Qed.
Lemma andNb b : ~~ b && b = false. Proof. by case: b. Qed.
Lemma orbN b : b || ~~ b = true.   Proof. by case: b. Qed.
Lemma orNb b : ~~ b || b = true.   Proof. by case: b. Qed.

Lemma andb_orl : left_distributive andb orb.  Proof. by do 3!case. Qed.
Lemma andb_orr : right_distributive andb orb. Proof. by do 3!case. Qed.
Lemma orb_andl : left_distributive orb andb.  Proof. by do 3!case. Qed.
Lemma orb_andr : right_distributive orb andb. Proof. by do 3!case. Qed.

Lemma andb_idl (a b : bool) : (b -> a) -> a && b = b.
Proof. by case: a; case: b => // ->. Qed.
Lemma andb_idr (a b : bool) : (a -> b) -> a && b = a.
Proof. by case: a; case: b => // ->. Qed.
Lemma andb_id2l (a b c : bool) : (a -> b = c) -> a && b = a && c.
Proof. by case: a; case: b; case: c => // ->. Qed.
Lemma andb_id2r (a b c : bool) : (b -> a = c) -> a && b = c && b.
Proof. by case: a; case: b; case: c => // ->. Qed.

Lemma orb_idl (a b : bool) : (a -> b) -> a || b = b.
Proof. by case: a; case: b => // ->. Qed.
Lemma orb_idr (a b : bool) : (b -> a) -> a || b = a.
Proof. by case: a; case: b => // ->. Qed.
Lemma orb_id2l (a b c : bool) : (~~ a -> b = c) -> a || b = a || c.
Proof. by case: a; case: b; case: c => // ->. Qed.
Lemma orb_id2r (a b c : bool) : (~~ b -> a = c) -> a || b = c || b.
Proof. by case: a; case: b; case: c => // ->. Qed.

Lemma negb_and (a b : bool) : ~~ (a && b) = ~~ a || ~~ b.
Proof. by case: a; case: b. Qed.

Lemma negb_or (a b : bool) : ~~ (a || b) = ~~ a && ~~ b.
Proof. by case: a; case: b. Qed.

(* Pseudo-cancellation -- i.e, absorbtion *)

Lemma andbK a b : a && b || a = a.  Proof. by case: a; case: b. Qed.
Lemma andKb a b : a || b && a = a.  Proof. by case: a; case: b. Qed.
Lemma orbK a b : (a || b) && a = a. Proof. by case: a; case: b. Qed.
Lemma orKb a b : a && (b || a) = a. Proof. by case: a; case: b. Qed.

(* Imply *)

Lemma implybT b : b ==> true.           Proof. by case: b. Qed.
Lemma implybF b : (b ==> false) = ~~ b. Proof. by case: b. Qed.
Lemma implyFb b : false ==> b.          Proof. by []. Qed.
Lemma implyTb b : (true ==> b) = b.     Proof. by []. Qed.
Lemma implybb b : b ==> b.              Proof. by case: b. Qed.

Lemma negb_imply a b : ~~ (a ==> b) = a && ~~ b.
Proof. by case: a; case: b. Qed.

Lemma implybE a b : (a ==> b) = ~~ a || b.
Proof. by case: a; case: b. Qed.

Lemma implyNb a b : (~~ a ==> b) = a || b.
Proof. by case: a; case: b. Qed.

Lemma implybN a b : (a ==> ~~ b) = (b ==> ~~ a).
Proof. by case: a; case: b. Qed.

Lemma implybNN a b : (~~ a ==> ~~ b) = b ==> a.
Proof. by case: a; case: b. Qed.

Lemma implyb_idl (a b : bool) : (~~ a -> b) -> (a ==> b) = b.
Proof. by case: a; case: b => // ->. Qed.
Lemma implyb_idr (a b : bool) : (b -> ~~ a) -> (a ==> b) = ~~ a.
Proof. by case: a; case: b => // ->. Qed.
Lemma implyb_id2l (a b c : bool) : (a -> b = c) -> (a ==> b) = (a ==> c).
Proof. by case: a; case: b; case: c => // ->. Qed.

(* Addition (xor) *)

Lemma addFb : left_id false addb.               Proof. by []. Qed.
Lemma addbF : right_id false addb.              Proof. by case. Qed.
Lemma addbb : self_inverse false addb.          Proof. by case. Qed.
Lemma addbC : commutative addb.                 Proof. by do 2!case. Qed.
Lemma addbA : associative addb.                 Proof. by do 3!case. Qed.
Lemma addbCA : left_commutative addb.           Proof. by do 3!case. Qed.
Lemma addbAC : right_commutative addb.          Proof. by do 3!case. Qed.
Lemma addbACA : interchange addb addb.          Proof. by do 4!case. Qed.
Lemma andb_addl : left_distributive andb addb.  Proof. by do 3!case. Qed.
Lemma andb_addr : right_distributive andb addb. Proof. by do 3!case. Qed.
Lemma addKb : left_loop id addb.                Proof. by do 2!case. Qed.
Lemma addbK : right_loop id addb.               Proof. by do 2!case. Qed.
Lemma addIb : left_injective addb.              Proof. by do 3!case. Qed.
Lemma addbI : right_injective addb.             Proof. by do 3!case. Qed.

Lemma addTb b : true (+) b = ~~ b. Proof. by []. Qed.
Lemma addbT b : b (+) true = ~~ b. Proof. by case: b. Qed.

Lemma addbN a b : a (+) ~~ b = ~~ (a (+) b).
Proof. by case: a; case: b. Qed.
Lemma addNb a b : ~~ a (+) b = ~~ (a (+) b).
Proof. by case: a; case: b. Qed.

Lemma addbP a b : reflect (~~ a = b) (a (+) b).
Proof. by case: a; case: b; constructor. Qed.
Implicit Arguments addbP [a b].

(* Resolution tactic for blindly weeding out common terms from boolean       *)
(* equalities. When faced with a goal of the form (andb/orb/addb b1 b2) = b3 *)
(* they will try to locate b1 in b3 and remove it. This can fail!            *)

Ltac bool_congr :=
  match goal with
  | |- (?X1 && ?X2 = ?X3) => first
  [ symmetry; rewrite -1?(andbC X1) -?(andbCA X1); congr 1 (andb X1); symmetry
  | case: (X1); [ rewrite ?andTb ?andbT // | by rewrite ?andbF /= ] ]
  | |- (?X1 || ?X2 = ?X3) => first
  [ symmetry; rewrite -1?(orbC X1) -?(orbCA X1); congr 1 (orb X1); symmetry
  | case: (X1); [ by rewrite ?orbT //= | rewrite ?orFb ?orbF ] ]
  | |- (?X1 (+) ?X2 = ?X3) =>
    symmetry; rewrite -1?(addbC X1) -?(addbCA X1); congr 1 (addb X1); symmetry
  | |- (~~ ?X1 = ?X2) => congr 1 negb
  end.

(******************************************************************************)
(* Predicates, i.e., packaged functions to bool.                              *)
(* - pred T, the basic type for predicates over a type T, is simply an alias  *)
(* for T -> bool.                                                             *)
(* We actually distinguish two kinds of predicates, which we call applicative *)
(* and collective, based on the syntax used to test them at some x in T:      *)
(* - For an applicative predicate P, one uses prefix syntax:                  *)
(*     P x                                                                    *)
(*   Also, most operations on applicative predicates use prefix syntax as     *)
(*   well (e.g., predI P Q).                                                  *)
(* - For a collective predicate A, one uses infix syntax:                     *)
(*     x \in A                                                                *)
(*   and all operations on collective predicates use infix syntax as well     *)
(*   (e.g., [predI A & B]).                                                   *)
(* There are only two kinds of applicative predicates:                        *)
(* - pred T, the alias for T -> bool mentioned above                          *)
(* - simpl_pred T, an alias for simpl_fun T bool with a coercion to pred T    *)
(*   that auto-simplifies on application (see ssrfun).                        *)
(* On the other hand, the set of collective predicate types is open-ended via *)
(* - predType T, a Structure that can be used to put Canonical collective     *)
(*   predicate interpretation on other types, such as lists, tuples,          *)
(*   finite sets, etc.                                                        *)
(* Indeed, we define such interpretations for applicative predicate types,    *)
(* which can therefore also be used with the infix syntax, e.g.,              *)
(*     x \in predI P Q                                                        *)
(* Moreover these infix forms are convertible to their prefix counterpart     *)
(* (e.g., predI P Q x which in turn simplifies to P x && Q x). The converse   *)
(* is not true, however; collective predicate types cannot, in general, be    *)
(* general, be used applicatively, because of the "uniform inheritance"       *)
(* restriction on implicit coercions.                                         *)
(*   However, we do define an explicit generic coercion                       *)
(* - mem : forall (pT : predType), pT -> mem_pred T                           *)
(*   where mem_pred T is a variant of simpl_pred T that preserves the infix   *)
(*   syntax, i.e., mem A x auto-simplifies to x \in A.                        *)
(* Indeed, the infix "collective" operators are notation for a prefix         *)
(* operator with arguments of type mem_pred T or pred T, applied to coerced   *)
(* collective predicates, e.g.,                                               *)
(*      Notation "x \in A" := (in_mem x (mem A)).                             *)
(* This prevents the variability in the predicate type from interfering with  *)
(* the application of generic lemmas. Moreover this also makes it much easier *)
(* to define generic lemmas, because the simplest type -- pred T -- can be    *)
(* used as the type of generic collective predicates, provided one takes care *)
(* not to use it applicatively; this avoids the burden of having to declare a *)
(* different predicate type for each predicate parameter of each section or   *)
(* lemma.                                                                     *)
(*   This trick is made possible by the fact that the constructor of the      *)
(* mem_pred T type aligns the unification process, forcing a generic          *)
(* "collective" predicate A : pred T to unify with the actual collective B,   *)
(* which mem has coerced to pred T via an internal, hidden implicit coercion, *)
(* supplied by the predType structure for B. Users should take care not to    *)
(* inadvertently "strip" (mem B) down to the coerced B, since this will       *)
(* expose the internal coercion: Coq will display a term B x that cannot be   *)
(* typed as such. The topredE lemma can be used to restore the x \in B        *)
(* syntax in this case. While -topredE can conversely be used to change       *)
(* x \in P into P x, it is safer to use the inE and memE lemmas instead, as   *)
(* they do not run the risk of exposing internal coercions. As a consequence  *)
(* it is better to explicitly cast a generic applicative pred T to simpl_pred *)
(* using the SimplPred constructor, when it is used as a collective predicate *)
(* (see, e.g., Lemma eq_big in bigop).                                        *)
(*   We also sometimes "instantiate" the predType structure by defining a     *)
(* coercion to the sort of the predPredType structure. This works better for  *)
(* types such as {set T} that have subtypes that coerce to them, since the    *)
(* same coercion will be inserted by the application of mem. It also lets us  *)
(* turn any Type aT : predArgType into the total predicate over that type,    *)
(* i.e., fun _: aT => true. This allows us to write, e.g., #|'I_n| for the    *)
(* cardinal of the (finite) type of integers less than n.                     *)
(*   Collective predicates have a specific extensional equality,              *)
(*   - A =i B,                                                                *)
(* while applicative predicates use the extensional equality of functions,    *)
(*   - P =1 Q                                                                 *)
(* The two forms are convertible, however.                                    *)
(* We lift boolean operations to predicates, defining:                        *)
(* - predU (union), predI (intersection), predC (complement),                 *)
(*   predD (difference), and preim (preimage, i.e., composition)              *)
(* For each operation we define three forms, typically:                       *)
(* - predU : pred T -> pred T -> simpl_pred T                                 *)
(* - [predU A & B], a Notation for predU (mem A) (mem B)                      *)
(* - xpredU, a Notation for the lambda-expression inside predU,               *)
(*     which is mostly useful as an argument of =1, since it exposes the head *)
(*     head constant of the expression to the ssreflect matching algorithm.   *)
(* The syntax for the preimage of a collective predicate A is                 *)
(* - [preim f of A]                                                           *)
(* Finally, the generic syntax for defining a simpl_pred T is                 *)
(* - [pred x : T | P(x)], [pred x | P(x)], [pred x in A | P(x)], etc.         *)
(* We also support boolean relations, but only the applicative form, with     *)
(* types                                                                      *)
(* - rel T, an alias for T -> pred T                                          *)
(* - simpl_rel T, an auto-simplifying version, and syntax                     *)
(*   [rel x y | P(x,y)], [rel x y in A & B | P(x,y)], etc.                    *)
(* The notation [rel of fA] can be used to coerce a function returning a      *)
(* collective predicate to one returning pred T.                              *)
(*   Finally, note that there is specific support for ambivalent predicates   *)
(* that can work in either style, as per this file's head descriptor.         *)
(******************************************************************************)

Definition pred T := T -> bool.

Identity Coercion fun_of_pred : pred >-> Funclass.

Definition rel T := T -> pred T.

Identity Coercion fun_of_rel : rel >-> Funclass.

Notation xpred0 := (fun _ => false).
Notation xpredT := (fun _ => true).
Notation xpredI := (fun (p1 p2 : pred _) x => p1 x && p2 x).
Notation xpredU := (fun (p1 p2 : pred _) x => p1 x || p2 x).
Notation xpredC := (fun (p : pred _) x => ~~ p x).
Notation xpredD := (fun (p1 p2 : pred _) x => ~~ p2 x && p1 x).
Notation xpreim := (fun f (p : pred _) x => p (f x)).
Notation xrelU := (fun (r1 r2 : rel _) x y => r1 x y || r2 x y).

Section Predicates.

Variables T : Type.

Definition subpred (p1 p2 : pred T) := forall x, p1 x -> p2 x.

Definition subrel (r1 r2 : rel T) := forall x y, r1 x y -> r2 x y.

Definition simpl_pred := simpl_fun T bool.
Definition applicative_pred := pred T.
Definition collective_pred := pred T.

Definition SimplPred (p : pred T) : simpl_pred := SimplFun p.

Coercion pred_of_simpl (p : simpl_pred) : pred T := fun_of_simpl p.
Coercion applicative_pred_of_simpl (p : simpl_pred) : applicative_pred :=
  fun_of_simpl p.
Coercion collective_pred_of_simpl (p : simpl_pred) : collective_pred :=
  fun x => (let: SimplFun f := p in fun _ => f x) x.
(* Note: applicative_of_simpl is convertible to pred_of_simpl, while *)
(* collective_of_simpl is not. *)

Definition pred0 := SimplPred xpred0.
Definition predT := SimplPred xpredT.
Definition predI p1 p2 := SimplPred (xpredI p1 p2).
Definition predU p1 p2 := SimplPred (xpredU p1 p2).
Definition predC p := SimplPred (xpredC p).
Definition predD p1 p2 := SimplPred (xpredD p1 p2).
Definition preim rT f (d : pred rT) := SimplPred (xpreim f d).

Definition simpl_rel := simpl_fun T (pred T).

Definition SimplRel (r : rel T) : simpl_rel := [fun x => r x].

Coercion rel_of_simpl_rel (r : simpl_rel) : rel T := fun x y => r x y.

Definition relU r1 r2 := SimplRel (xrelU r1 r2).

Lemma subrelUl r1 r2 : subrel r1 (relU r1 r2).
Proof. by move=> *; apply/orP; left. Qed.

Lemma subrelUr r1 r2 : subrel r2 (relU r1 r2).
Proof. by move=> *; apply/orP; right. Qed.

Variant mem_pred := Mem of pred T.

Definition isMem pT topred mem := mem = (fun p : pT => Mem [eta topred p]).

Structure predType := PredType {
  pred_sort :> Type;
  topred : pred_sort -> pred T;
  _ : {mem | isMem topred mem}
}.

Definition mkPredType pT toP := PredType (exist (@isMem pT toP) _ (erefl _)).

Canonical predPredType := Eval hnf in @mkPredType (pred T) id.
Canonical simplPredType := Eval hnf in mkPredType pred_of_simpl.
Canonical boolfunPredType := Eval hnf in @mkPredType (T -> bool) id.

Coercion pred_of_mem mp : pred_sort predPredType := let: Mem p := mp in [eta p].
Canonical memPredType := Eval hnf in mkPredType pred_of_mem.

Definition clone_pred U :=
  fun pT & pred_sort pT -> U =>
  fun a mP (pT' := @PredType U a mP) & phant_id pT' pT => pT'.

End Predicates.

Implicit Arguments pred0 [T].
Implicit Arguments predT [T].
Prenex Implicits pred0 predT predI predU predC predD preim relU.

Notation "[ 'pred' : T | E ]" := (SimplPred (fun _ : T => E%B))
  (at level 0, format "[ 'pred' :  T  |  E ]") : fun_scope.
Notation "[ 'pred' x | E ]" := (SimplPred (fun x => E%B))
  (at level 0, x ident, format "[ 'pred'  x  |  E ]") : fun_scope.
Notation "[ 'pred' x | E1 & E2 ]" := [pred x | E1 && E2 ]
  (at level 0, x ident, format "[ 'pred'  x  |  E1  &  E2 ]") : fun_scope.
Notation "[ 'pred' x : T | E ]" := (SimplPred (fun x : T => E%B))
  (at level 0, x ident, only parsing) : fun_scope.
Notation "[ 'pred' x : T | E1 & E2 ]" := [pred x : T | E1 && E2 ]
  (at level 0, x ident, only parsing) : fun_scope.
Notation "[ 'rel' x y | E ]" := (SimplRel (fun x y => E%B))
  (at level 0, x ident, y ident, format "[ 'rel'  x  y  |  E ]") : fun_scope.
Notation "[ 'rel' x y : T | E ]" := (SimplRel (fun x y : T => E%B))
  (at level 0, x ident, y ident, only parsing) : fun_scope.

Notation "[ 'predType' 'of' T ]" := (@clone_pred _ T _ id _ _ id)
  (at level 0, format "[ 'predType'  'of'  T ]") : form_scope.

(* This redundant coercion lets us "inherit" the simpl_predType canonical    *)
(* instance by declaring a coercion to simpl_pred. This hack is the only way *)
(* to put a predType structure on a predArgType. We use simpl_pred rather    *)
(* than pred to ensure that /= removes the identity coercion. Note that the  *)
(* coercion will never be used directly for simpl_pred, since the canonical  *)
(* instance should always be resolved.                                       *)

Notation pred_class := (pred_sort (predPredType _)).
Coercion sort_of_simpl_pred T (p : simpl_pred T) : pred_class := p : pred T.

(* This lets us use some types as a synonym for their universal predicate.    *)
(* Unfortunately, this won't work for existing types like bool, unless we     *)
(* redefine bool, true, false and all bool ops.                               *)
Definition predArgType := Type.
Bind Scope type_scope with predArgType.
Identity Coercion sort_of_predArgType : predArgType >-> Sortclass.
Coercion pred_of_argType (T : predArgType) : simpl_pred T := predT.

Notation "{ : T }" := (T%type : predArgType)
  (at level 0, format "{ :  T }") : type_scope.

(* These must be defined outside a Section because "cooking" kills the        *)
(* nosimpl tag.                                                               *)

Definition mem T (pT : predType T) : pT -> mem_pred T :=
  nosimpl (let: PredType _ _ (exist mem _) := pT return pT -> _ in mem).
Definition in_mem T x mp := nosimpl pred_of_mem T mp x.

Prenex Implicits mem.

Coercion pred_of_mem_pred T mp := [pred x : T | in_mem x mp].

Definition eq_mem T p1 p2 := forall x : T, in_mem x p1 = in_mem x p2.
Definition sub_mem T p1 p2 := forall x : T, in_mem x p1 -> in_mem x p2.

Typeclasses Opaque eq_mem.

Lemma sub_refl T (p : mem_pred T) : sub_mem p p. Proof. by []. Qed.
Implicit Arguments sub_refl [[T] [p]].

Notation "x \in A" := (in_mem x (mem A)) : bool_scope.
Notation "x \in A" := (in_mem x (mem A)) : bool_scope.
Notation "x \notin A" := (~~ (x \in A)) : bool_scope.
Notation "A =i B" := (eq_mem (mem A) (mem B)) : type_scope.
Notation "{ 'subset' A <= B }" := (sub_mem (mem A) (mem B))
  (at level 0, A, B at level 69,
   format "{ '[hv' 'subset'  A '/   '  <=  B ']' }") : type_scope.
Notation "[ 'mem' A ]" := (pred_of_simpl (pred_of_mem_pred (mem A)))
  (at level 0, only parsing) : fun_scope.
Notation "[ 'rel' 'of' fA ]" := (fun x => [mem (fA x)])
  (at level 0, format "[ 'rel'  'of'  fA ]") : fun_scope.
Notation "[ 'predI' A & B ]" := (predI [mem A] [mem B])
  (at level 0, format "[ 'predI'  A  &  B ]") : fun_scope.
Notation "[ 'predU' A & B ]" := (predU [mem A] [mem B])
  (at level 0, format "[ 'predU'  A  &  B ]") : fun_scope.
Notation "[ 'predD' A & B ]" := (predD [mem A] [mem B])
  (at level 0, format "[ 'predD'  A  &  B ]") : fun_scope.
Notation "[ 'predC' A ]" := (predC [mem A])
  (at level 0, format "[ 'predC'  A ]") : fun_scope.
Notation "[ 'preim' f 'of' A ]" := (preim f [mem A])
  (at level 0, format "[ 'preim'  f  'of'  A ]") : fun_scope.

Notation "[ 'pred' x 'in' A ]" := [pred x | x \in A]
  (at level 0, x ident, format "[ 'pred'  x  'in'  A ]") : fun_scope.
Notation "[ 'pred' x 'in' A | E ]" := [pred x | x \in A & E]
  (at level 0, x ident, format "[ 'pred'  x  'in'  A  |  E ]") : fun_scope.
Notation "[ 'pred' x 'in' A | E1 & E2 ]" := [pred x | x \in A & E1 && E2 ]
  (at level 0, x ident,
   format "[ 'pred'  x  'in'  A  |  E1  &  E2 ]") : fun_scope.
Notation "[ 'rel' x y 'in' A & B | E ]" :=
  [rel x y | (x \in A) && (y \in B) && E]
  (at level 0, x ident, y ident,
   format "[ 'rel'  x  y  'in'  A  &  B  |  E ]") : fun_scope.
Notation "[ 'rel' x y 'in' A & B ]" := [rel x y | (x \in A) && (y \in B)]
  (at level 0, x ident, y ident,
   format "[ 'rel'  x  y  'in'  A  &  B ]") : fun_scope.
Notation "[ 'rel' x y 'in' A | E ]" := [rel x y in A & A | E]
  (at level 0, x ident, y ident,
   format "[ 'rel'  x  y  'in'  A  |  E ]") : fun_scope.
Notation "[ 'rel' x y 'in' A ]" := [rel x y in A & A]
  (at level 0, x ident, y ident,
   format "[ 'rel'  x  y  'in'  A ]") : fun_scope.

Section simpl_mem.

Variables (T : Type) (pT : predType T).
Implicit Types (x : T) (p : pred T) (sp : simpl_pred T) (pp : pT).

(* Bespoke structures that provide fine-grained control over matching the     *)
(* various forms of the \in predicate; note in particular the different forms *)
(* of hoisting that are used. We had to work around several bugs in the       *)
(* implementation of unification, notably improper expansion of telescope     *)
(* projections and overwriting of a variable assignment by a later            *)
(* unification (probably due to conversion cache cross-talk).                 *)
Structure manifest_applicative_pred p := ManifestApplicativePred {
  manifest_applicative_pred_value :> pred T;
  _ : manifest_applicative_pred_value = p
}.
Definition ApplicativePred p := ManifestApplicativePred (erefl p).
Canonical applicative_pred_applicative sp :=
  ApplicativePred (applicative_pred_of_simpl sp).

Structure manifest_simpl_pred p := ManifestSimplPred {
  manifest_simpl_pred_value :> simpl_pred T;
  _ : manifest_simpl_pred_value = SimplPred p
}.
Canonical expose_simpl_pred p := ManifestSimplPred (erefl (SimplPred p)).

Structure manifest_mem_pred p := ManifestMemPred {
  manifest_mem_pred_value :> mem_pred T;
  _ : manifest_mem_pred_value= Mem [eta p]
}.
Canonical expose_mem_pred p :=  @ManifestMemPred p _ (erefl _).

Structure applicative_mem_pred p :=
  ApplicativeMemPred {applicative_mem_pred_value :> manifest_mem_pred p}.
Canonical check_applicative_mem_pred p (ap : manifest_applicative_pred p) mp :=
  @ApplicativeMemPred ap mp.

Lemma mem_topred (pp : pT) : mem (topred pp) = mem pp.
Proof. by rewrite /mem; case: pT pp => T1 app1 [mem1 /= ->]. Qed.

Lemma topredE x (pp : pT) : topred pp x = (x \in pp).
Proof. by rewrite -mem_topred. Qed.

Lemma app_predE x p (ap : manifest_applicative_pred p) : ap x = (x \in p).
Proof. by case: ap => _ /= ->. Qed.

Lemma in_applicative x p (amp : applicative_mem_pred p) : in_mem x amp = p x.
Proof. by case: amp => [[_ /= ->]]. Qed.

Lemma in_collective x p (msp : manifest_simpl_pred p) :
  (x \in collective_pred_of_simpl msp) = p x.
Proof. by case: msp => _ /= ->. Qed.

Lemma in_simpl x p (msp : manifest_simpl_pred p) :
  in_mem x (Mem [eta fun_of_simpl (msp : simpl_pred T)]) = p x.
Proof. by case: msp => _ /= ->. Qed.

(* Because of the explicit eta expansion in the left-hand side, this lemma    *)
(* should only be used in a right-to-left direction. The 8.3 hack allowing    *)
(* partial right-to-left use does not work with the improved expansion        *)
(* heuristics in 8.4.                                                         *)
Lemma unfold_in x p : (x \in ([eta p] : pred T)) = p x.
Proof. by []. Qed.

Lemma simpl_predE p : SimplPred p =1 p.
Proof. by []. Qed.

Definition inE := (in_applicative, in_simpl, simpl_predE). (* to be extended *)

Lemma mem_simpl sp : mem sp = sp :> pred T.
Proof. by []. Qed.

Definition memE := mem_simpl. (* could be extended *)

Lemma mem_mem (pp : pT) : (mem (mem pp) = mem pp) * (mem [mem pp] = mem pp).
Proof. by rewrite -mem_topred. Qed.

End simpl_mem.

(* Qualifiers and keyed predicates. *)

Variant qualifier (q : nat) T := Qualifier of predPredType T.

Coercion has_quality n T (q : qualifier n T) : pred_class :=
  fun x => let: Qualifier p := q in p x.
Implicit Arguments has_quality [T].

Lemma qualifE n T p x : (x \in @Qualifier n T p) = p x. Proof. by []. Qed.

Notation "x \is A" := (x \in has_quality 0 A)
  (at level 70, no associativity,
   format "'[hv' x '/ '  \is  A ']'") : bool_scope.
Notation "x \is 'a' A" := (x \in has_quality 1 A)
  (at level 70, no associativity,
   format "'[hv' x '/ '  \is  'a'  A ']'") : bool_scope.
Notation "x \is 'an' A" := (x \in has_quality 2 A)
  (at level 70, no associativity,
   format "'[hv' x '/ ' \is  'an'  A ']'") : bool_scope.
Notation "x \isn't A" := (x \notin has_quality 0 A)
  (at level 70, no associativity,
   format "'[hv' x '/ '  \isn't  A ']'") : bool_scope.
Notation "x \isn't 'a' A" := (x \notin has_quality 1 A)
  (at level 70, no associativity,
   format "'[hv' x '/ '  \isn't  'a'  A ']'") : bool_scope.
Notation "x \isn't 'an' A" := (x \notin has_quality 2 A)
  (at level 70, no associativity,
   format "'[hv' x '/ ' \isn't  'an'  A ']'") : bool_scope.
Notation "[ 'qualify' x | P ]" := (Qualifier 0 (fun x => P%B))
  (at level 0, x at level 99,
   format "'[hv' [  'qualify'  x  | '/ '  P ] ']'") : form_scope.
Notation "[ 'qualify' x : T | P ]" := (Qualifier 0 (fun x : T => P%B))
  (at level 0, x at level 99, only parsing) : form_scope.
Notation "[ 'qualify' 'a' x | P ]" := (Qualifier 1 (fun x => P%B))
  (at level 0, x at level 99,
   format "'[hv' [ 'qualify'  'a'  x  | '/ '  P ] ']'") : form_scope.
Notation "[ 'qualify' 'a' x : T | P ]" := (Qualifier 1 (fun x : T => P%B))
  (at level 0, x at level 99, only parsing) : form_scope.
Notation "[ 'qualify' 'an' x | P ]" := (Qualifier 2 (fun x => P%B))
  (at level 0, x at level 99,
   format "'[hv' [ 'qualify'  'an'  x  | '/ '  P ] ']'") : form_scope.
Notation "[ 'qualify' 'an' x : T | P ]" := (Qualifier 2 (fun x : T => P%B))
  (at level 0, x at level 99, only parsing) : form_scope.

(* Keyed predicates: support for property-bearing predicate interfaces. *)

Section KeyPred.

Variable T : Type.
Variant pred_key (p : predPredType T) := DefaultPredKey.

Variable p : predPredType T.
Structure keyed_pred (k : pred_key p) :=
  PackKeyedPred {unkey_pred :> pred_class; _ : unkey_pred =i p}.

Variable k : pred_key p.
Definition KeyedPred := @PackKeyedPred k p (frefl _).

Variable k_p : keyed_pred k.
Lemma keyed_predE : k_p =i p. Proof. by case: k_p. Qed.

(* Instances that strip the mem cast; the first one has "pred_of_mem" as its  *)
(* projection head value, while the second has "pred_of_simpl". The latter    *)
(* has the side benefit of preempting accidental misdeclarations.             *)
(* Note: pred_of_mem is the registered mem >-> pred_class coercion, while     *)
(* simpl_of_mem; pred_of_simpl is the mem >-> pred >=> Funclass coercion. We  *)
(* must write down the coercions explicitly as the Canonical head constant    *)
(* computation does not strip casts !!                                        *)
Canonical keyed_mem :=
  @PackKeyedPred k (pred_of_mem (mem k_p)) keyed_predE.
Canonical keyed_mem_simpl :=
  @PackKeyedPred k (pred_of_simpl (mem k_p)) keyed_predE.

End KeyPred.

Notation "x \i 'n' S" := (x \in @unkey_pred _ S _ _)
  (at level 70, format "'[hv' x '/ '  \i 'n'  S ']'") : bool_scope.

Section KeyedQualifier.

Variables (T : Type) (n : nat) (q : qualifier n T).

Structure keyed_qualifier (k : pred_key q) :=
  PackKeyedQualifier {unkey_qualifier; _ : unkey_qualifier = q}.
Definition KeyedQualifier k := PackKeyedQualifier k (erefl q).
Variables (k : pred_key q) (k_q : keyed_qualifier k).
Fact keyed_qualifier_suproof : unkey_qualifier k_q =i q.
Proof. by case: k_q => /= _ ->. Qed.
Canonical keyed_qualifier_keyed := PackKeyedPred k keyed_qualifier_suproof.

End KeyedQualifier.

Notation "x \i 's' A" := (x \i n has_quality 0 A)
  (at level 70, format "'[hv' x '/ '  \i 's'  A ']'") : bool_scope.
Notation "x \i 's' 'a' A" := (x \i n has_quality 1 A)
  (at level 70, format "'[hv' x '/ '  \i 's'  'a'  A ']'") : bool_scope.
Notation "x \i 's' 'an' A" := (x \i n has_quality 2 A)
  (at level 70, format "'[hv' x '/ '  \i 's'  'an'  A ']'") : bool_scope.

Module DefaultKeying.

Canonical default_keyed_pred T p := KeyedPred (@DefaultPredKey T p).
Canonical default_keyed_qualifier T n (q : qualifier n T) :=
  KeyedQualifier (DefaultPredKey q).

End DefaultKeying.

(* Skolemizing with conditions. *)

Lemma all_tag_cond_dep I T (C : pred I) U :
    (forall x, T x) -> (forall x, C x -> {y : T x & U x y}) ->
  {f : forall x, T x & forall x, C x -> U x (f x)}.
Proof.
move=> f0 fP; apply: all_tag (fun x y => C x -> U x y) _ => x.
by case Cx: (C x); [case/fP: Cx => y; exists y | exists (f0 x)].
Qed.

Lemma all_tag_cond I T (C : pred I) U :
    T -> (forall x, C x -> {y : T & U x y}) ->
  {f : I -> T & forall x, C x -> U x (f x)}.
Proof. by move=> y0; apply: all_tag_cond_dep. Qed.

Lemma all_sig_cond_dep I T (C : pred I) P :
    (forall x, T x) -> (forall x, C x -> {y : T x | P x y}) ->
  {f : forall x, T x | forall x, C x -> P x (f x)}.
Proof. by move=> f0 /(all_tag_cond_dep f0)[f]; exists f. Qed.

Lemma all_sig_cond I T (C : pred I) P :
    T -> (forall x, C x -> {y : T | P x y}) ->
  {f : I -> T | forall x, C x -> P x (f x)}.
Proof. by move=> y0; apply: all_sig_cond_dep. Qed.

Section RelationProperties.

(* Caveat: reflexive should not be used to state lemmas, as auto and trivial  *)
(* will not expand the constant.                                              *)

Variable T : Type.

Variable R : rel T.

Definition total := forall x y, R x y || R y x.
Definition transitive := forall y x z, R x y -> R y z -> R x z.

Definition symmetric := forall x y, R x y = R y x.
Definition antisymmetric := forall x y, R x y && R y x -> x = y.
Definition pre_symmetric := forall x y, R x y -> R y x.

Lemma symmetric_from_pre : pre_symmetric -> symmetric.
Proof. by move=> symR x y; apply/idP/idP; apply: symR. Qed.

Definition reflexive := forall x, R x x.
Definition irreflexive := forall x, R x x = false.

Definition left_transitive := forall x y, R x y -> R x =1 R y.
Definition right_transitive := forall x y, R x y -> R^~ x =1 R^~ y.

Section PER.

Hypotheses (symR : symmetric) (trR : transitive).

Lemma sym_left_transitive : left_transitive.
Proof. by move=> x y Rxy z; apply/idP/idP; apply: trR; rewrite // symR. Qed.

Lemma sym_right_transitive : right_transitive.
Proof. by move=> x y /sym_left_transitive Rxy z; rewrite !(symR z) Rxy. Qed.

End PER.

(* We define the equivalence property with prenex quantification so that it   *)
(* can be localized using the {in ..., ..} form defined below.                *)

Definition equivalence_rel := forall x y z, R z z * (R x y -> R x z = R y z).

Lemma equivalence_relP : equivalence_rel <-> reflexive /\ left_transitive.
Proof.
split=> [eqiR | [Rxx trR] x y z]; last by split=> [|/trR->].
by split=> [x | x y Rxy z]; [rewrite (eqiR x x x) | rewrite (eqiR x y z)].
Qed.

End RelationProperties.

Lemma rev_trans T (R : rel T) : transitive R -> transitive (fun x y => R y x).
Proof. by move=> trR x y z Ryx Rzy; apply: trR Rzy Ryx. Qed.

(* Property localization *)

Local Notation "{ 'all1' P }" := (forall x, P x : Prop) (at level 0).
Local Notation "{ 'all2' P }" := (forall x y, P x y : Prop) (at level 0).
Local Notation "{ 'all3' P }" := (forall x y z, P x y z: Prop) (at level 0).
Local Notation ph := (phantom _).

Section LocalProperties.

Variables T1 T2 T3 : Type.

Variables (d1 : mem_pred T1) (d2 : mem_pred T2) (d3 : mem_pred T3).
Local Notation ph := (phantom Prop).

Definition prop_for (x : T1) P & ph {all1 P} := P x.

Lemma forE x P phP : @prop_for x P phP = P x. Proof. by []. Qed.

Definition prop_in1 P & ph {all1 P} :=
  forall x, in_mem x d1 -> P x.

Definition prop_in11 P & ph {all2 P} :=
  forall x y, in_mem x d1 -> in_mem y d2 -> P x y.

Definition prop_in2 P & ph {all2 P} :=
  forall x y, in_mem x d1 -> in_mem y d1 -> P x y.

Definition prop_in111 P & ph {all3 P} :=
  forall x y z, in_mem x d1 -> in_mem y d2 -> in_mem z d3 -> P x y z.

Definition prop_in12 P & ph {all3 P} :=
  forall x y z, in_mem x d1 -> in_mem y d2 -> in_mem z d2 -> P x y z.

Definition prop_in21 P & ph {all3 P} :=
  forall x y z, in_mem x d1 -> in_mem y d1 -> in_mem z d2 -> P x y z.

Definition prop_in3 P & ph {all3 P} :=
  forall x y z, in_mem x d1 -> in_mem y d1 -> in_mem z d1 -> P x y z.

Variable f : T1 -> T2.

Definition prop_on1 Pf P & phantom T3 (Pf f) & ph {all1 P} :=
  forall x, in_mem (f x) d2 -> P x.

Definition prop_on2 Pf P & phantom T3 (Pf f) & ph {all2 P} :=
  forall x y, in_mem (f x) d2 -> in_mem (f y) d2 -> P x y.

End LocalProperties.

Definition inPhantom := Phantom Prop.
Definition onPhantom T P (x : T) := Phantom Prop (P x).

Definition bijective_in aT rT (d : mem_pred aT) (f : aT -> rT) :=
  exists2 g, prop_in1 d (inPhantom (cancel f g))
           & prop_on1 d (Phantom _ (cancel g)) (onPhantom (cancel g) f).

Definition bijective_on aT rT (cd : mem_pred rT) (f : aT -> rT) :=
  exists2 g, prop_on1 cd (Phantom _ (cancel f)) (onPhantom (cancel f) g)
           & prop_in1 cd (inPhantom (cancel g f)).

Notation "{ 'for' x , P }" :=
  (prop_for x (inPhantom P))
  (at level 0, format "{ 'for'  x ,  P }") : type_scope.

Notation "{ 'in' d , P }" :=
  (prop_in1 (mem d) (inPhantom P))
  (at level 0, format "{ 'in'  d ,  P }") : type_scope.

Notation "{ 'in' d1 & d2 , P }" :=
  (prop_in11 (mem d1) (mem d2) (inPhantom P))
  (at level 0, format "{ 'in'  d1  &  d2 ,  P }") : type_scope.

Notation "{ 'in' d & , P }" :=
  (prop_in2 (mem d) (inPhantom P))
  (at level 0, format "{ 'in'  d  & ,  P }") : type_scope.

Notation "{ 'in' d1 & d2 & d3 , P }" :=
  (prop_in111 (mem d1) (mem d2) (mem d3) (inPhantom P))
  (at level 0, format "{ 'in'  d1  &  d2  &  d3 ,  P }") : type_scope.

Notation "{ 'in' d1 & & d3 , P }" :=
  (prop_in21 (mem d1) (mem d3) (inPhantom P))
  (at level 0, format "{ 'in'  d1  &  &  d3 ,  P }") : type_scope.

Notation "{ 'in' d1 & d2 & , P }" :=
  (prop_in12 (mem d1) (mem d2) (inPhantom P))
  (at level 0, format "{ 'in'  d1  &  d2  & ,  P }") : type_scope.

Notation "{ 'in' d & & , P }" :=
  (prop_in3 (mem d) (inPhantom P))
  (at level 0, format "{ 'in'  d  &  & ,  P }") : type_scope.

Notation "{ 'on' cd , P }" :=
  (prop_on1 (mem cd) (inPhantom P) (inPhantom P))
  (at level 0, format "{ 'on'  cd ,  P }") : type_scope.

Notation "{ 'on' cd & , P }" :=
  (prop_on2 (mem cd) (inPhantom P) (inPhantom P))
  (at level 0, format "{ 'on'  cd  & ,  P }") : type_scope.

Local Arguments onPhantom {_%type_scope} _ _.

Notation "{ 'on' cd , P & g }" :=
  (prop_on1 (mem cd) (Phantom (_ -> Prop) P) (onPhantom P g))
  (at level 0, format "{ 'on'  cd ,  P  &  g }") : type_scope.

Notation "{ 'in' d , 'bijective' f }" := (bijective_in (mem d) f)
  (at level 0, f at level 8,
   format "{ 'in'  d ,  'bijective'  f }") : type_scope.

Notation "{ 'on' cd , 'bijective' f }" := (bijective_on (mem cd) f)
  (at level 0, f at level 8,
   format "{ 'on'  cd ,  'bijective'  f }") : type_scope.

(* Weakening and monotonicity lemmas for localized predicates.                *)
(* Note that using these lemmas in backward reasoning will force expansion of *)
(* the predicate definition, as Coq needs to expose the quantifier to apply   *)
(* these lemmas. We define a few specialized variants to avoid this for some  *)
(* of the ssrfun predicates.                                                  *)

Section LocalGlobal.

Variables T1 T2 T3 : predArgType.
Variables (D1 : pred T1) (D2 : pred T2) (D3 : pred T3).
Variables (d1 d1' : mem_pred T1) (d2 d2' : mem_pred T2) (d3 d3' : mem_pred T3).
Variables (f f' : T1 -> T2) (g : T2 -> T1) (h : T3).
Variables (P1 : T1 -> Prop) (P2 : T1 -> T2 -> Prop).
Variable P3 : T1 -> T2 -> T3 -> Prop.
Variable Q1 : (T1 -> T2) -> T1 -> Prop.
Variable Q1l : (T1 -> T2) -> T3 -> T1 -> Prop.
Variable Q2 : (T1 -> T2) -> T1 -> T1 -> Prop.

Hypothesis sub1 : sub_mem d1 d1'.
Hypothesis sub2 : sub_mem d2 d2'.
Hypothesis sub3 : sub_mem d3 d3'.

Lemma in1W : {all1 P1} -> {in D1, {all1 P1}}.
Proof. by move=> ? ?. Qed.
Lemma in2W : {all2 P2} -> {in D1 & D2, {all2 P2}}.
Proof. by move=> ? ?. Qed.
Lemma in3W : {all3 P3} -> {in D1 & D2 & D3, {all3 P3}}.
Proof. by move=> ? ?. Qed.

Lemma in1T : {in T1, {all1 P1}} -> {all1 P1}.
Proof. by move=> ? ?; auto. Qed.
Lemma in2T : {in T1 & T2, {all2 P2}} -> {all2 P2}.
Proof. by move=> ? ?; auto. Qed.
Lemma in3T : {in T1 & T2 & T3, {all3 P3}} -> {all3 P3}.
Proof. by move=> ? ?; auto. Qed.

Lemma sub_in1 (Ph : ph {all1 P1}) : prop_in1 d1' Ph -> prop_in1 d1 Ph.
Proof. by move=> allP x /sub1; apply: allP. Qed.

Lemma sub_in11 (Ph : ph {all2 P2}) : prop_in11 d1' d2' Ph -> prop_in11 d1 d2 Ph.
Proof. by move=> allP x1 x2 /sub1 d1x1 /sub2; apply: allP. Qed.

Lemma sub_in111 (Ph : ph {all3 P3}) :
  prop_in111 d1' d2' d3' Ph -> prop_in111 d1 d2 d3 Ph.
Proof. by move=> allP x1 x2 x3 /sub1 d1x1 /sub2 d2x2 /sub3; apply: allP. Qed.

Let allQ1 f'' := {all1 Q1 f''}.
Let allQ1l f'' h' := {all1 Q1l f'' h'}.
Let allQ2 f'' := {all2 Q2 f''}.

Lemma on1W : allQ1 f -> {on D2, allQ1 f}. Proof. by move=> ? ?. Qed.

Lemma on1lW : allQ1l f h -> {on D2, allQ1l f & h}. Proof. by move=> ? ?. Qed.

Lemma on2W : allQ2 f -> {on D2 &, allQ2 f}. Proof. by move=> ? ?. Qed.

Lemma on1T : {on T2, allQ1 f} -> allQ1 f. Proof. by move=> ? ?; auto. Qed.

Lemma on1lT : {on T2, allQ1l f & h} -> allQ1l f h.
Proof. by move=> ? ?; auto. Qed.

Lemma on2T : {on T2 &, allQ2 f} -> allQ2 f.
Proof. by move=> ? ?; auto. Qed.

Lemma subon1 (Phf : ph (allQ1 f)) (Ph : ph (allQ1 f)) :
  prop_on1 d2' Phf Ph -> prop_on1 d2 Phf Ph.
Proof. by move=> allQ x /sub2; apply: allQ. Qed.

Lemma subon1l (Phf : ph (allQ1l f)) (Ph : ph (allQ1l f h)) :
  prop_on1 d2' Phf Ph -> prop_on1 d2 Phf Ph.
Proof. by move=> allQ x /sub2; apply: allQ. Qed.

Lemma subon2 (Phf : ph (allQ2 f)) (Ph : ph (allQ2 f)) :
  prop_on2 d2' Phf Ph -> prop_on2 d2 Phf Ph.
Proof. by move=> allQ x y /sub2=> d2fx /sub2; apply: allQ. Qed.

Lemma can_in_inj : {in D1, cancel f g} -> {in D1 &, injective f}.
Proof. by move=> fK x y /fK{2}<- /fK{2}<- ->. Qed.

Lemma canLR_in x y : {in D1, cancel f g} -> y \in D1 -> x = f y -> g x = y.
Proof. by move=> fK D1y ->; rewrite fK. Qed.

Lemma canRL_in x y : {in D1, cancel f g} -> x \in D1 -> f x = y -> x = g y.
Proof. by move=> fK D1x <-; rewrite fK. Qed.

Lemma on_can_inj : {on D2, cancel f & g} -> {on D2 &, injective f}.
Proof. by move=> fK x y /fK{2}<- /fK{2}<- ->. Qed.

Lemma canLR_on x y : {on D2, cancel f & g} -> f y \in D2 -> x = f y -> g x = y.
Proof. by move=> fK D2fy ->; rewrite fK. Qed.

Lemma canRL_on x y : {on D2, cancel f & g} -> f x \in D2 -> f x = y -> x = g y.
Proof. by move=> fK D2fx <-; rewrite fK. Qed.

Lemma inW_bij : bijective f -> {in D1, bijective f}.
Proof. by case=> g' fK g'K; exists g' => * ? *; auto. Qed.

Lemma onW_bij : bijective f -> {on D2, bijective f}.
Proof. by case=> g' fK g'K; exists g' => * ? *; auto. Qed.

Lemma inT_bij : {in T1, bijective f} -> bijective f.
Proof. by case=> g' fK g'K; exists g' => * ? *; auto. Qed.

Lemma onT_bij : {on T2, bijective f} -> bijective f.
Proof. by case=> g' fK g'K; exists g' => * ? *; auto. Qed.

Lemma sub_in_bij (D1' : pred T1) :
  {subset D1 <= D1'} -> {in D1', bijective f} -> {in D1, bijective f}.
Proof.
by move=> subD [g' fK g'K]; exists g' => x; move/subD; [apply: fK | apply: g'K].
Qed.

Lemma subon_bij (D2' : pred T2) :
  {subset D2 <= D2'} -> {on D2', bijective f} -> {on D2, bijective f}.
Proof.
by move=> subD [g' fK g'K]; exists g' => x; move/subD; [apply: fK | apply: g'K].
Qed.

End LocalGlobal.

Lemma sub_in2 T d d' (P : T -> T -> Prop) :
  sub_mem d d' -> forall Ph : ph {all2 P}, prop_in2 d' Ph -> prop_in2 d Ph.
Proof. by move=> /= sub_dd'; apply: sub_in11. Qed.

Lemma sub_in3 T d d' (P : T -> T -> T -> Prop) :
  sub_mem d d' -> forall Ph : ph {all3 P}, prop_in3 d' Ph -> prop_in3 d Ph.
Proof. by move=> /= sub_dd'; apply: sub_in111. Qed.

Lemma sub_in12 T1 T d1 d1' d d' (P : T1 -> T -> T -> Prop) :
  sub_mem d1 d1' -> sub_mem d d' ->
  forall Ph : ph {all3 P}, prop_in12 d1' d' Ph -> prop_in12 d1 d Ph.
Proof. by move=> /= sub1 sub; apply: sub_in111. Qed.

Lemma sub_in21 T T3 d d' d3 d3' (P : T -> T -> T3 -> Prop) :
  sub_mem d d' -> sub_mem d3 d3' ->
  forall Ph : ph {all3 P}, prop_in21 d' d3' Ph -> prop_in21 d d3 Ph.
Proof. by move=> /= sub sub3; apply: sub_in111. Qed.

Lemma equivalence_relP_in T (R : rel T) (A : pred T) :
  {in A & &, equivalence_rel R}
   <-> {in A, reflexive R} /\ {in A &, forall x y, R x y -> {in A, R x =1 R y}}.
Proof.
split=> [eqiR | [Rxx trR] x y z *]; last by split=> [|/trR-> //]; apply: Rxx.
by split=> [x Ax|x y Ax Ay Rxy z Az]; [rewrite (eqiR x x) | rewrite (eqiR x y)].
Qed.

Section MonoHomoMorphismTheory.

Variables (aT rT sT : Type) (f : aT -> rT) (g : rT -> aT).
Variables (aP : pred aT) (rP : pred rT) (aR : rel aT) (rR : rel rT).

Lemma monoW : {mono f : x / aP x >-> rP x} -> {homo f : x / aP x >-> rP x}.
Proof. by move=> hf x ax; rewrite hf. Qed.

Lemma mono2W :
  {mono f : x y / aR x y >-> rR x y} -> {homo f : x y / aR x y >-> rR x y}.
Proof. by move=> hf x y axy; rewrite hf. Qed.

Hypothesis fgK : cancel g f.

Lemma homoRL :
  {homo f : x y / aR x y >-> rR x y} -> forall x y, aR (g x) y -> rR x (f y).
Proof. by move=> Hf x y /Hf; rewrite fgK. Qed.

Lemma homoLR :
  {homo f : x y / aR x y >-> rR x y} -> forall x y, aR x (g y) -> rR (f x) y.
Proof. by move=> Hf x y /Hf; rewrite fgK. Qed.

Lemma homo_mono :
    {homo f : x y / aR x y >-> rR x y} -> {homo g : x y / rR x y >-> aR x y} ->
  {mono g : x y / rR x y >-> aR x y}.
Proof.
move=> mf mg x y; case: (boolP (rR _ _))=> [/mg //|].
by apply: contraNF=> /mf; rewrite !fgK.
Qed.

Lemma monoLR :
  {mono f : x y / aR x y >-> rR x y} -> forall x y, rR (f x) y = aR x (g y).
Proof. by move=> mf x y; rewrite -{1}[y]fgK mf. Qed.

Lemma monoRL :
  {mono f : x y / aR x y >-> rR x y} -> forall x y, rR x (f y) = aR (g x) y.
Proof. by move=> mf x y; rewrite -{1}[x]fgK mf. Qed.

Lemma can_mono :
  {mono f : x y / aR x y >-> rR x y} -> {mono g : x y / rR x y >-> aR x y}.
Proof. by move=> mf x y /=; rewrite -mf !fgK. Qed.

End MonoHomoMorphismTheory.

Section MonoHomoMorphismTheory_in.

Variables (aT rT sT : predArgType) (f : aT -> rT) (g : rT -> aT).
Variable (aD : pred aT).
Variable (aP : pred aT) (rP : pred rT) (aR : rel aT) (rR : rel rT).

Notation rD := [pred x | g x \in aD].

Lemma monoW_in :
    {in aD &, {mono f : x y / aR x y >-> rR x y}} ->
  {in aD &, {homo f : x y / aR x y >-> rR x y}}.
Proof. by move=> hf x y hx hy axy; rewrite hf. Qed.

Lemma mono2W_in :
    {in aD, {mono f : x / aP x >-> rP x}} ->
  {in aD, {homo f : x / aP x >-> rP x}}.
Proof. by move=> hf x hx ax; rewrite hf. Qed.

Hypothesis fgK_on : {on aD, cancel g & f}.

Lemma homoRL_in :
    {in aD &, {homo f : x y / aR x y >-> rR x y}} ->
  {in rD & aD, forall x y, aR (g x) y -> rR x (f y)}.
Proof. by move=> Hf x y hx hy /Hf; rewrite fgK_on //; apply. Qed.

Lemma homoLR_in :
    {in aD &, {homo f : x y / aR x y >-> rR x y}} ->
  {in aD & rD, forall x y, aR x (g y) -> rR (f x) y}.
Proof. by move=> Hf x y hx hy /Hf; rewrite fgK_on //; apply. Qed.

Lemma homo_mono_in :
    {in aD &, {homo f : x y / aR x y >-> rR x y}} ->
    {in rD &, {homo g : x y / rR x y >-> aR x y}} ->
  {in rD &, {mono g : x y / rR x y >-> aR x y}}.
Proof.
move=> mf mg x y hx hy; case: (boolP (rR _ _))=> [/mg //|]; first exact.
by apply: contraNF=> /mf; rewrite !fgK_on //; apply.
Qed.

Lemma monoLR_in :
    {in aD &, {mono f : x y / aR x y >-> rR x y}} ->
  {in aD & rD, forall x y, rR (f x) y = aR x (g y)}.
Proof. by move=> mf x y hx hy; rewrite -{1}[y]fgK_on // mf. Qed.

Lemma monoRL_in :
    {in aD &, {mono f : x y / aR x y >-> rR x y}} ->
  {in rD & aD, forall x y, rR x (f y) = aR (g x) y}.
Proof. by move=> mf x y hx hy; rewrite -{1}[x]fgK_on // mf. Qed.

Lemma can_mono_in :
    {in aD &, {mono f : x y / aR x y >-> rR x y}} ->
  {in rD &, {mono g : x y / rR x y >-> aR x y}}.
Proof. by move=> mf x y hx hy /=; rewrite -mf // !fgK_on. Qed.

End MonoHomoMorphismTheory_in.
