# Theory Sublist

theory Sublist
imports Main
```(*  Title:      HOL/Library/Sublist.thy
Author:     Tobias Nipkow and Markus Wenzel, TU München
Author:     Christian Sternagel, JAIST
Author:     Manuel Eberl, TU München
*)

section ‹List prefixes, suffixes, and homeomorphic embedding›

theory Sublist
imports Main
begin

subsection ‹Prefix order on lists›

definition prefix :: "'a list ⇒ 'a list ⇒ bool"
where "prefix xs ys ⟷ (∃zs. ys = xs @ zs)"

definition strict_prefix :: "'a list ⇒ 'a list ⇒ bool"
where "strict_prefix xs ys ⟷ prefix xs ys ∧ xs ≠ ys"

interpretation prefix_order: order prefix strict_prefix
by standard (auto simp: prefix_def strict_prefix_def)

interpretation prefix_bot: order_bot Nil prefix strict_prefix

lemma prefixI [intro?]: "ys = xs @ zs ⟹ prefix xs ys"
unfolding prefix_def by blast

lemma prefixE [elim?]:
assumes "prefix xs ys"
obtains zs where "ys = xs @ zs"
using assms unfolding prefix_def by blast

lemma strict_prefixI' [intro?]: "ys = xs @ z # zs ⟹ strict_prefix xs ys"
unfolding strict_prefix_def prefix_def by blast

lemma strict_prefixE' [elim?]:
assumes "strict_prefix xs ys"
obtains z zs where "ys = xs @ z # zs"
proof -
from ‹strict_prefix xs ys› obtain us where "ys = xs @ us" and "xs ≠ ys"
unfolding strict_prefix_def prefix_def by blast
with that show ?thesis by (auto simp add: neq_Nil_conv)
qed

(* FIXME rm *)
lemma strict_prefixI [intro?]: "prefix xs ys ⟹ xs ≠ ys ⟹ strict_prefix xs ys"
by(fact prefix_order.le_neq_trans)

lemma strict_prefixE [elim?]:
fixes xs ys :: "'a list"
assumes "strict_prefix xs ys"
obtains "prefix xs ys" and "xs ≠ ys"
using assms unfolding strict_prefix_def by blast

subsection ‹Basic properties of prefixes›

(* FIXME rm *)
theorem Nil_prefix [simp]: "prefix [] xs"
by (fact prefix_bot.bot_least)

(* FIXME rm *)
theorem prefix_Nil [simp]: "(prefix xs []) = (xs = [])"
by (fact prefix_bot.bot_unique)

lemma prefix_snoc [simp]: "prefix xs (ys @ [y]) ⟷ xs = ys @ [y] ∨ prefix xs ys"
proof
assume "prefix xs (ys @ [y])"
then obtain zs where zs: "ys @ [y] = xs @ zs" ..
show "xs = ys @ [y] ∨ prefix xs ys"
by (metis append_Nil2 butlast_append butlast_snoc prefixI zs)
next
assume "xs = ys @ [y] ∨ prefix xs ys"
then show "prefix xs (ys @ [y])"
by (metis prefix_order.eq_iff prefix_order.order_trans prefixI)
qed

lemma Cons_prefix_Cons [simp]: "prefix (x # xs) (y # ys) = (x = y ∧ prefix xs ys)"

lemma prefix_code [code]:
"prefix [] xs ⟷ True"
"prefix (x # xs) [] ⟷ False"
"prefix (x # xs) (y # ys) ⟷ x = y ∧ prefix xs ys"
by simp_all

lemma same_prefix_prefix [simp]: "prefix (xs @ ys) (xs @ zs) = prefix ys zs"
by (induct xs) simp_all

lemma same_prefix_nil [simp]: "prefix (xs @ ys) xs = (ys = [])"
by (metis append_Nil2 append_self_conv prefix_order.eq_iff prefixI)

lemma prefix_prefix [simp]: "prefix xs ys ⟹ prefix xs (ys @ zs)"
unfolding prefix_def by fastforce

lemma append_prefixD: "prefix (xs @ ys) zs ⟹ prefix xs zs"

theorem prefix_Cons: "prefix xs (y # ys) = (xs = [] ∨ (∃zs. xs = y # zs ∧ prefix zs ys))"
by (cases xs) (auto simp add: prefix_def)

theorem prefix_append:
"prefix xs (ys @ zs) = (prefix xs ys ∨ (∃us. xs = ys @ us ∧ prefix us zs))"
apply (induct zs rule: rev_induct)
apply force
apply (simp flip: append_assoc)
apply (metis append_eq_appendI)
done

lemma append_one_prefix:
"prefix xs ys ⟹ length xs < length ys ⟹ prefix (xs @ [ys ! length xs]) ys"
proof (unfold prefix_def)
assume a1: "∃zs. ys = xs @ zs"
then obtain sk :: "'a list" where sk: "ys = xs @ sk" by fastforce
assume a2: "length xs < length ys"
have f1: "⋀v. ([]::'a list) @ v = v" using append_Nil2 by simp
have "[] ≠ sk" using a1 a2 sk less_not_refl by force
hence "∃v. xs @ hd sk # v = ys" using sk by (metis hd_Cons_tl)
thus "∃zs. ys = (xs @ [ys ! length xs]) @ zs" using f1 by fastforce
qed

theorem prefix_length_le: "prefix xs ys ⟹ length xs ≤ length ys"

lemma prefix_same_cases:
"prefix (xs⇩1::'a list) ys ⟹ prefix xs⇩2 ys ⟹ prefix xs⇩1 xs⇩2 ∨ prefix xs⇩2 xs⇩1"
unfolding prefix_def by (force simp: append_eq_append_conv2)

lemma prefix_length_prefix:
"prefix ps xs ⟹ prefix qs xs ⟹ length ps ≤ length qs ⟹ prefix ps qs"
by (auto simp: prefix_def) (metis append_Nil2 append_eq_append_conv_if)

lemma set_mono_prefix: "prefix xs ys ⟹ set xs ⊆ set ys"

lemma take_is_prefix: "prefix (take n xs) xs"
unfolding prefix_def by (metis append_take_drop_id)

lemma prefixeq_butlast: "prefix (butlast xs) xs"

lemma prefix_map_rightE:
assumes "prefix xs (map f ys)"
shows   "∃xs'. prefix xs' ys ∧ xs = map f xs'"
proof -
define n where "n = length xs"
have "xs = take n (map f ys)"
using assms by (auto simp: prefix_def n_def)
thus ?thesis
by (intro exI[of _ "take n ys"]) (auto simp: take_map take_is_prefix)
qed

lemma map_mono_prefix: "prefix xs ys ⟹ prefix (map f xs) (map f ys)"
by (auto simp: prefix_def)

lemma filter_mono_prefix: "prefix xs ys ⟹ prefix (filter P xs) (filter P ys)"
by (auto simp: prefix_def)

lemma sorted_antimono_prefix: "prefix xs ys ⟹ sorted ys ⟹ sorted xs"
by (metis sorted_append prefix_def)

lemma prefix_length_less: "strict_prefix xs ys ⟹ length xs < length ys"
by (auto simp: strict_prefix_def prefix_def)

lemma prefix_snocD: "prefix (xs@[x]) ys ⟹ strict_prefix xs ys"

lemma strict_prefix_simps [simp, code]:
"strict_prefix xs [] ⟷ False"
"strict_prefix [] (x # xs) ⟷ True"
"strict_prefix (x # xs) (y # ys) ⟷ x = y ∧ strict_prefix xs ys"
by (simp_all add: strict_prefix_def cong: conj_cong)

lemma take_strict_prefix: "strict_prefix xs ys ⟹ strict_prefix (take n xs) ys"
proof (induct n arbitrary: xs ys)
case 0
then show ?case by (cases ys) simp_all
next
case (Suc n)
then show ?case by (metis prefix_order.less_trans strict_prefixI take_is_prefix)
qed

lemma prefix_takeWhile:
assumes "prefix xs ys"
shows   "prefix (takeWhile P xs) (takeWhile P ys)"
proof -
from assms obtain zs where ys: "ys = xs @ zs"
by (auto simp: prefix_def)
have "prefix (takeWhile P xs) (takeWhile P (xs @ zs))"
by (induction xs) auto
thus ?thesis by (simp add: ys)
qed

lemma prefix_dropWhile:
assumes "prefix xs ys"
shows   "prefix (dropWhile P xs) (dropWhile P ys)"
proof -
from assms obtain zs where ys: "ys = xs @ zs"
by (auto simp: prefix_def)
have "prefix (dropWhile P xs) (dropWhile P (xs @ zs))"
by (induction xs) auto
thus ?thesis by (simp add: ys)
qed

assumes "prefix xs ys"
using assms
proof (induction "length xs" arbitrary: xs ys rule: less_induct)
case (less xs)
show ?case
proof (cases xs)
case [simp]: (Cons x xs')
then obtain y ys' where [simp]: "ys = y # ys'"
using ‹prefix xs ys› by (cases ys) auto
from less show ?thesis
by (auto simp: remdups_adj_Cons' less_Suc_eq_le length_dropWhile_le
intro!: less prefix_dropWhile)
qed auto
qed

lemma not_prefix_cases:
assumes pfx: "¬ prefix ps ls"
obtains
(c1) "ps ≠ []" and "ls = []"
| (c2) a as x xs where "ps = a#as" and "ls = x#xs" and "x = a" and "¬ prefix as xs"
| (c3) a as x xs where "ps = a#as" and "ls = x#xs" and "x ≠ a"
proof (cases ps)
case Nil
then show ?thesis using pfx by simp
next
case (Cons a as)
note c = ‹ps = a#as›
show ?thesis
proof (cases ls)
case Nil then show ?thesis by (metis append_Nil2 pfx c1 same_prefix_nil)
next
case (Cons x xs)
show ?thesis
proof (cases "x = a")
case True
have "¬ prefix as xs" using pfx c Cons True by simp
with c Cons True show ?thesis by (rule c2)
next
case False
with c Cons show ?thesis by (rule c3)
qed
qed
qed

lemma not_prefix_induct [consumes 1, case_names Nil Neq Eq]:
assumes np: "¬ prefix ps ls"
and base: "⋀x xs. P (x#xs) []"
and r1: "⋀x xs y ys. x ≠ y ⟹ P (x#xs) (y#ys)"
and r2: "⋀x xs y ys. ⟦ x = y; ¬ prefix xs ys; P xs ys ⟧ ⟹ P (x#xs) (y#ys)"
shows "P ps ls" using np
proof (induct ls arbitrary: ps)
case Nil
then show ?case
by (auto simp: neq_Nil_conv elim!: not_prefix_cases intro!: base)
next
case (Cons y ys)
then have npfx: "¬ prefix ps (y # ys)" by simp
then obtain x xs where pv: "ps = x # xs"
by (rule not_prefix_cases) auto
show ?case by (metis Cons.hyps Cons_prefix_Cons npfx pv r1 r2)
qed

subsection ‹Prefixes›

primrec prefixes where
"prefixes [] = [[]]" |
"prefixes (x#xs) = [] # map ((#) x) (prefixes xs)"

lemma in_set_prefixes[simp]: "xs ∈ set (prefixes ys) ⟷ prefix xs ys"
proof (induct xs arbitrary: ys)
case Nil
then show ?case by (cases ys) auto
next
case (Cons a xs)
then show ?case by (cases ys) auto
qed

lemma length_prefixes[simp]: "length (prefixes xs) = length xs+1"
by (induction xs) auto

lemma distinct_prefixes [intro]: "distinct (prefixes xs)"
by (induction xs) (auto simp: distinct_map)

lemma prefixes_snoc [simp]: "prefixes (xs@[x]) = prefixes xs @ [xs@[x]]"
by (induction xs) auto

lemma prefixes_not_Nil [simp]: "prefixes xs ≠ []"
by (cases xs) auto

lemma hd_prefixes [simp]: "hd (prefixes xs) = []"
by (cases xs) simp_all

lemma last_prefixes [simp]: "last (prefixes xs) = xs"
by (induction xs) (simp_all add: last_map)

lemma prefixes_append:
"prefixes (xs @ ys) = prefixes xs @ map (λys'. xs @ ys') (tl (prefixes ys))"
proof (induction xs)
case Nil
thus ?case by (cases ys) auto
qed simp_all

lemma prefixes_eq_snoc:
"prefixes ys = xs @ [x] ⟷
(ys = [] ∧ xs = [] ∨ (∃z zs. ys = zs@[z] ∧ xs = prefixes zs)) ∧ x = ys"
by (cases ys rule: rev_cases) auto

lemma prefixes_tailrec [code]:
"prefixes xs = rev (snd (foldl (λ(acc1, acc2) x. (x#acc1, rev (x#acc1)#acc2)) ([],[[]]) xs))"
proof -
have "foldl (λ(acc1, acc2) x. (x#acc1, rev (x#acc1)#acc2)) (ys, rev ys # zs) xs =
(rev xs @ ys, rev (map (λas. rev ys @ as) (prefixes xs)) @ zs)" for ys zs
proof (induction xs arbitrary: ys zs)
case (Cons x xs ys zs)
from Cons.IH[of "x # ys" "rev ys # zs"]
show ?case by (simp add: o_def)
qed simp_all
from this [of "[]" "[]"] show ?thesis by simp
qed

lemma set_prefixes_eq: "set (prefixes xs) = {ys. prefix ys xs}"
by auto

lemma card_set_prefixes [simp]: "card (set (prefixes xs)) = Suc (length xs)"
by (subst distinct_card) auto

lemma set_prefixes_append:
"set (prefixes (xs @ ys)) = set (prefixes xs) ∪ {xs @ ys' |ys'. ys' ∈ set (prefixes ys)}"
by (subst prefixes_append, cases ys) auto

subsection ‹Longest Common Prefix›

definition Longest_common_prefix :: "'a list set ⇒ 'a list" where
"Longest_common_prefix L = (ARG_MAX length ps. ∀xs ∈ L. prefix ps xs)"

lemma Longest_common_prefix_ex: "L ≠ {} ⟹
∃ps. (∀xs ∈ L. prefix ps xs) ∧ (∀qs. (∀xs ∈ L. prefix qs xs) ⟶ size qs ≤ size ps)"
(is "_ ⟹ ∃ps. ?P L ps")
proof(induction "LEAST n. ∃xs ∈L. n = length xs" arbitrary: L)
case 0
have "[] ∈ L" using "0.hyps" LeastI[of "λn. ∃xs∈L. n = length xs"] ‹L ≠ {}›
by auto
hence "?P L []" by(auto)
thus ?case ..
next
case (Suc n)
let ?EX = "λn. ∃xs∈L. n = length xs"
obtain x xs where xxs: "x#xs ∈ L" "size xs = n" using Suc.prems Suc.hyps(2)
by(metis LeastI_ex[of ?EX] Suc_length_conv ex_in_conv)
hence "[] ∉ L" using Suc.hyps(2) by auto
show ?case
proof (cases "∀xs ∈ L. ∃ys. xs = x#ys")
case True
let ?L = "{ys. x#ys ∈ L}"
have 1: "(LEAST n. ∃xs ∈ ?L. n = length xs) = n"
using xxs Suc.prems Suc.hyps(2) Least_le[of "?EX"]
by - (rule Least_equality, fastforce+)
have 2: "?L ≠ {}" using ‹x # xs ∈ L› by auto
from Suc.hyps(1)[OF 1[symmetric] 2] obtain ps where IH: "?P ?L ps" ..
{ fix qs
assume "∀qs. (∀xa. x # xa ∈ L ⟶ prefix qs xa) ⟶ length qs ≤ length ps"
and "∀xs∈L. prefix qs xs"
hence "length (tl qs) ≤ length ps"
by (metis Cons_prefix_Cons hd_Cons_tl list.sel(2) Nil_prefix)
hence "length qs ≤ Suc (length ps)" by auto
}
hence "?P L (x#ps)" using True IH by auto
thus ?thesis ..
next
case False
then obtain y ys where yys: "x≠y" "y#ys ∈ L" using ‹[] ∉ L›
by (auto) (metis list.exhaust)
have "∀qs. (∀xs∈L. prefix qs xs) ⟶ qs = []" using yys ‹x#xs ∈ L›
by auto (metis Cons_prefix_Cons prefix_Cons)
hence "?P L []" by auto
thus ?thesis ..
qed
qed

lemma Longest_common_prefix_unique: "L ≠ {} ⟹
∃! ps. (∀xs ∈ L. prefix ps xs) ∧ (∀qs. (∀xs ∈ L. prefix qs xs) ⟶ size qs ≤ size ps)"
by(rule ex_ex1I[OF Longest_common_prefix_ex];
meson equals0I prefix_length_prefix prefix_order.antisym)

lemma Longest_common_prefix_eq:
"⟦ L ≠ {};  ∀xs ∈ L. prefix ps xs;
∀qs. (∀xs ∈ L. prefix qs xs) ⟶ size qs ≤ size ps ⟧
⟹ Longest_common_prefix L = ps"
unfolding Longest_common_prefix_def arg_max_def is_arg_max_linorder
by(rule some1_equality[OF Longest_common_prefix_unique]) auto

lemma Longest_common_prefix_prefix:
"xs ∈ L ⟹ prefix (Longest_common_prefix L) xs"
unfolding Longest_common_prefix_def arg_max_def is_arg_max_linorder
by(rule someI2_ex[OF Longest_common_prefix_ex]) auto

lemma Longest_common_prefix_longest:
"L ≠ {} ⟹ ∀xs∈L. prefix ps xs ⟹ length ps ≤ length(Longest_common_prefix L)"
unfolding Longest_common_prefix_def arg_max_def is_arg_max_linorder
by(rule someI2_ex[OF Longest_common_prefix_ex]) auto

lemma Longest_common_prefix_max_prefix:
"L ≠ {} ⟹ ∀xs∈L. prefix ps xs ⟹ prefix ps (Longest_common_prefix L)"
by(metis Longest_common_prefix_prefix Longest_common_prefix_longest
prefix_length_prefix ex_in_conv)

lemma Longest_common_prefix_Nil: "[] ∈ L ⟹ Longest_common_prefix L = []"
using Longest_common_prefix_prefix prefix_Nil by blast

lemma Longest_common_prefix_image_Cons: "L ≠ {} ⟹
Longest_common_prefix ((#) x ` L) = x # Longest_common_prefix L"
apply(rule Longest_common_prefix_eq)
apply(simp)
apply simp
by(metis Longest_common_prefix_longest[of L] Cons_prefix_Cons Nitpick.size_list_simp(2)
Suc_le_mono hd_Cons_tl order.strict_implies_order zero_less_Suc)

lemma Longest_common_prefix_eq_Cons: assumes "L ≠ {}" "[] ∉ L"  "∀xs∈L. hd xs = x"
shows "Longest_common_prefix L = x # Longest_common_prefix {ys. x#ys ∈ L}"
proof -
have "L = (#) x ` {ys. x#ys ∈ L}" using assms(2,3)
by (auto simp: image_def)(metis hd_Cons_tl)
thus ?thesis
by (metis Longest_common_prefix_image_Cons image_is_empty assms(1))
qed

lemma Longest_common_prefix_eq_Nil:
"⟦x#ys ∈ L; y#zs ∈ L; x ≠ y ⟧ ⟹ Longest_common_prefix L = []"
by (metis Longest_common_prefix_prefix list.inject prefix_Cons)

fun longest_common_prefix :: "'a list ⇒ 'a list ⇒ 'a list" where
"longest_common_prefix (x#xs) (y#ys) =
(if x=y then x # longest_common_prefix xs ys else [])" |
"longest_common_prefix _ _ = []"

lemma longest_common_prefix_prefix1:
"prefix (longest_common_prefix xs ys) xs"
by(induction xs ys rule: longest_common_prefix.induct) auto

lemma longest_common_prefix_prefix2:
"prefix (longest_common_prefix xs ys) ys"
by(induction xs ys rule: longest_common_prefix.induct) auto

lemma longest_common_prefix_max_prefix:
"⟦ prefix ps xs; prefix ps ys ⟧
⟹ prefix ps (longest_common_prefix xs ys)"
by(induction xs ys arbitrary: ps rule: longest_common_prefix.induct)
(auto simp: prefix_Cons)

subsection ‹Parallel lists›

definition parallel :: "'a list ⇒ 'a list ⇒ bool"  (infixl "∥" 50)
where "(xs ∥ ys) = (¬ prefix xs ys ∧ ¬ prefix ys xs)"

lemma parallelI [intro]: "¬ prefix xs ys ⟹ ¬ prefix ys xs ⟹ xs ∥ ys"
unfolding parallel_def by blast

lemma parallelE [elim]:
assumes "xs ∥ ys"
obtains "¬ prefix xs ys ∧ ¬ prefix ys xs"
using assms unfolding parallel_def by blast

theorem prefix_cases:
obtains "prefix xs ys" | "strict_prefix ys xs" | "xs ∥ ys"
unfolding parallel_def strict_prefix_def by blast

theorem parallel_decomp:
"xs ∥ ys ⟹ ∃as b bs c cs. b ≠ c ∧ xs = as @ b # bs ∧ ys = as @ c # cs"
proof (induct xs rule: rev_induct)
case Nil
then have False by auto
then show ?case ..
next
case (snoc x xs)
show ?case
proof (rule prefix_cases)
assume le: "prefix xs ys"
then obtain ys' where ys: "ys = xs @ ys'" ..
show ?thesis
proof (cases ys')
assume "ys' = []"
then show ?thesis by (metis append_Nil2 parallelE prefixI snoc.prems ys)
next
fix c cs assume ys': "ys' = c # cs"
have "x ≠ c" using snoc.prems ys ys' by fastforce
thus "∃as b bs c cs. b ≠ c ∧ xs @ [x] = as @ b # bs ∧ ys = as @ c # cs"
using ys ys' by blast
qed
next
assume "strict_prefix ys xs"
then have "prefix ys (xs @ [x])" by (simp add: strict_prefix_def)
with snoc have False by blast
then show ?thesis ..
next
assume "xs ∥ ys"
with snoc obtain as b bs c cs where neq: "(b::'a) ≠ c"
and xs: "xs = as @ b # bs" and ys: "ys = as @ c # cs"
by blast
from xs have "xs @ [x] = as @ b # (bs @ [x])" by simp
with neq ys show ?thesis by blast
qed
qed

lemma parallel_append: "a ∥ b ⟹ a @ c ∥ b @ d"
apply (rule parallelI)
apply (erule parallelE, erule conjE,
induct rule: not_prefix_induct, simp+)+
done

lemma parallel_appendI: "xs ∥ ys ⟹ x = xs @ xs' ⟹ y = ys @ ys' ⟹ x ∥ y"

lemma parallel_commute: "a ∥ b ⟷ b ∥ a"
unfolding parallel_def by auto

subsection ‹Suffix order on lists›

definition suffix :: "'a list ⇒ 'a list ⇒ bool"
where "suffix xs ys = (∃zs. ys = zs @ xs)"

definition strict_suffix :: "'a list ⇒ 'a list ⇒ bool"
where "strict_suffix xs ys ⟷ suffix xs ys ∧ xs ≠ ys"

interpretation suffix_order: order suffix strict_suffix
by standard (auto simp: suffix_def strict_suffix_def)

interpretation suffix_bot: order_bot Nil suffix strict_suffix

lemma suffixI [intro?]: "ys = zs @ xs ⟹ suffix xs ys"
unfolding suffix_def by blast

lemma suffixE [elim?]:
assumes "suffix xs ys"
obtains zs where "ys = zs @ xs"
using assms unfolding suffix_def by blast

lemma suffix_tl [simp]: "suffix (tl xs) xs"
by (induct xs) (auto simp: suffix_def)

lemma strict_suffix_tl [simp]: "xs ≠ [] ⟹ strict_suffix (tl xs) xs"
by (induct xs) (auto simp: strict_suffix_def suffix_def)

lemma Nil_suffix [simp]: "suffix [] xs"

lemma suffix_Nil [simp]: "(suffix xs []) = (xs = [])"

lemma suffix_ConsI: "suffix xs ys ⟹ suffix xs (y # ys)"

lemma suffix_ConsD: "suffix (x # xs) ys ⟹ suffix xs ys"

lemma suffix_appendI: "suffix xs ys ⟹ suffix xs (zs @ ys)"

lemma suffix_appendD: "suffix (zs @ xs) ys ⟹ suffix xs ys"

lemma strict_suffix_set_subset: "strict_suffix xs ys ⟹ set xs ⊆ set ys"
by (auto simp: strict_suffix_def suffix_def)

lemma set_mono_suffix: "suffix xs ys ⟹ set xs ⊆ set ys"
by (auto simp: suffix_def)

lemma sorted_antimono_suffix: "suffix xs ys ⟹ sorted ys ⟹ sorted xs"
by (metis sorted_append suffix_def)

lemma suffix_ConsD2: "suffix (x # xs) (y # ys) ⟹ suffix xs ys"
proof -
assume "suffix (x # xs) (y # ys)"
then obtain zs where "y # ys = zs @ x # xs" ..
then show ?thesis
by (induct zs) (auto intro!: suffix_appendI suffix_ConsI)
qed

lemma suffix_to_prefix [code]: "suffix xs ys ⟷ prefix (rev xs) (rev ys)"
proof
assume "suffix xs ys"
then obtain zs where "ys = zs @ xs" ..
then have "rev ys = rev xs @ rev zs" by simp
then show "prefix (rev xs) (rev ys)" ..
next
assume "prefix (rev xs) (rev ys)"
then obtain zs where "rev ys = rev xs @ zs" ..
then have "rev (rev ys) = rev zs @ rev (rev xs)" by simp
then have "ys = rev zs @ xs" by simp
then show "suffix xs ys" ..
qed

lemma strict_suffix_to_prefix [code]: "strict_suffix xs ys ⟷ strict_prefix (rev xs) (rev ys)"
by (auto simp: suffix_to_prefix strict_suffix_def strict_prefix_def)

lemma distinct_suffix: "distinct ys ⟹ suffix xs ys ⟹ distinct xs"
by (clarsimp elim!: suffixE)

lemma map_mono_suffix: "suffix xs ys ⟹ suffix (map f xs) (map f ys)"
by (auto elim!: suffixE intro: suffixI)

lemma filter_mono_suffix: "suffix xs ys ⟹ suffix (filter P xs) (filter P ys)"
by (auto simp: suffix_def)

lemma suffix_drop: "suffix (drop n as) as"
unfolding suffix_def by (rule exI [where x = "take n as"]) simp

lemma suffix_take: "suffix xs ys ⟹ ys = take (length ys - length xs) ys @ xs"
by (auto elim!: suffixE)

lemma strict_suffix_reflclp_conv: "strict_suffix⇧=⇧= = suffix"
by (intro ext) (auto simp: suffix_def strict_suffix_def)

lemma suffix_lists: "suffix xs ys ⟹ ys ∈ lists A ⟹ xs ∈ lists A"
unfolding suffix_def by auto

lemma suffix_snoc [simp]: "suffix xs (ys @ [y]) ⟷ xs = [] ∨ (∃zs. xs = zs @ [y] ∧ suffix zs ys)"
by (cases xs rule: rev_cases) (auto simp: suffix_def)

lemma snoc_suffix_snoc [simp]: "suffix (xs @ [x]) (ys @ [y]) = (x = y ∧ suffix xs ys)"

lemma same_suffix_suffix [simp]: "suffix (ys @ xs) (zs @ xs) = suffix ys zs"

lemma same_suffix_nil [simp]: "suffix (ys @ xs) xs = (ys = [])"

theorem suffix_Cons: "suffix xs (y # ys) ⟷ xs = y # ys ∨ suffix xs ys"
unfolding suffix_def by (auto simp: Cons_eq_append_conv)

theorem suffix_append:
"suffix xs (ys @ zs) ⟷ suffix xs zs ∨ (∃xs'. xs = xs' @ zs ∧ suffix xs' ys)"
by (auto simp: suffix_def append_eq_append_conv2)

theorem suffix_length_le: "suffix xs ys ⟹ length xs ≤ length ys"

lemma suffix_same_cases:
"suffix (xs⇩1::'a list) ys ⟹ suffix xs⇩2 ys ⟹ suffix xs⇩1 xs⇩2 ∨ suffix xs⇩2 xs⇩1"
unfolding suffix_def by (force simp: append_eq_append_conv2)

lemma suffix_length_suffix:
"suffix ps xs ⟹ suffix qs xs ⟹ length ps ≤ length qs ⟹ suffix ps qs"
by (auto simp: suffix_to_prefix intro: prefix_length_prefix)

lemma suffix_length_less: "strict_suffix xs ys ⟹ length xs < length ys"
by (auto simp: strict_suffix_def suffix_def)

lemma suffix_ConsD': "suffix (x#xs) ys ⟹ strict_suffix xs ys"
by (auto simp: strict_suffix_def suffix_def)

lemma drop_strict_suffix: "strict_suffix xs ys ⟹ strict_suffix (drop n xs) ys"
proof (induct n arbitrary: xs ys)
case 0
then show ?case by (cases ys) simp_all
next
case (Suc n)
then show ?case
by (cases xs) (auto intro: Suc dest: suffix_ConsD' suffix_order.less_imp_le)
qed

lemma suffix_map_rightE:
assumes "suffix xs (map f ys)"
shows   "∃xs'. suffix xs' ys ∧ xs = map f xs'"
proof -
from assms obtain xs' where xs': "map f ys = xs' @ xs"
by (auto simp: suffix_def)
define n where "n = length xs'"
have "xs = drop n (map f ys)"
thus ?thesis
by (intro exI[of _ "drop n ys"]) (auto simp: drop_map suffix_drop)
qed

using prefix_remdups_adj[of "rev xs" "rev ys"]

lemma not_suffix_cases:
assumes pfx: "¬ suffix ps ls"
obtains
(c1) "ps ≠ []" and "ls = []"
| (c2) a as x xs where "ps = as@[a]" and "ls = xs@[x]" and "x = a" and "¬ suffix as xs"
| (c3) a as x xs where "ps = as@[a]" and "ls = xs@[x]" and "x ≠ a"
proof (cases ps rule: rev_cases)
case Nil
then show ?thesis using pfx by simp
next
case (snoc as a)
note c = ‹ps = as@[a]›
show ?thesis
proof (cases ls rule: rev_cases)
case Nil then show ?thesis by (metis append_Nil2 pfx c1 same_suffix_nil)
next
case (snoc xs x)
show ?thesis
proof (cases "x = a")
case True
have "¬ suffix as xs" using pfx c snoc True by simp
with c snoc True show ?thesis by (rule c2)
next
case False
with c snoc show ?thesis by (rule c3)
qed
qed
qed

lemma not_suffix_induct [consumes 1, case_names Nil Neq Eq]:
assumes np: "¬ suffix ps ls"
and base: "⋀x xs. P (xs@[x]) []"
and r1: "⋀x xs y ys. x ≠ y ⟹ P (xs@[x]) (ys@[y])"
and r2: "⋀x xs y ys. ⟦ x = y; ¬ suffix xs ys; P xs ys ⟧ ⟹ P (xs@[x]) (ys@[y])"
shows "P ps ls" using np
proof (induct ls arbitrary: ps rule: rev_induct)
case Nil
then show ?case by (cases ps rule: rev_cases) (auto intro: base)
next
case (snoc y ys ps)
then have npfx: "¬ suffix ps (ys @ [y])" by simp
then obtain x xs where pv: "ps = xs @ [x]"
by (rule not_suffix_cases) auto
show ?case by (metis snoc.hyps snoc_suffix_snoc npfx pv r1 r2)
qed

lemma parallelD1: "x ∥ y ⟹ ¬ prefix x y"
by blast

lemma parallelD2: "x ∥ y ⟹ ¬ prefix y x"
by blast

lemma parallel_Nil1 [simp]: "¬ x ∥ []"
unfolding parallel_def by simp

lemma parallel_Nil2 [simp]: "¬ [] ∥ x"
unfolding parallel_def by simp

lemma Cons_parallelI1: "a ≠ b ⟹ a # as ∥ b # bs"
by auto

lemma Cons_parallelI2: "⟦ a = b; as ∥ bs ⟧ ⟹ a # as ∥ b # bs"
by (metis Cons_prefix_Cons parallelE parallelI)

lemma not_equal_is_parallel:
assumes neq: "xs ≠ ys"
and len: "length xs = length ys"
shows "xs ∥ ys"
using len neq
proof (induct rule: list_induct2)
case Nil
then show ?case by simp
next
case (Cons a as b bs)
have ih: "as ≠ bs ⟹ as ∥ bs" by fact
show ?case
proof (cases "a = b")
case True
then have "as ≠ bs" using Cons by simp
then show ?thesis by (rule Cons_parallelI2 [OF True ih])
next
case False
then show ?thesis by (rule Cons_parallelI1)
qed
qed

subsection ‹Suffixes›

primrec suffixes where
"suffixes [] = [[]]"
| "suffixes (x#xs) = suffixes xs @ [x # xs]"

lemma in_set_suffixes [simp]: "xs ∈ set (suffixes ys) ⟷ suffix xs ys"
by (induction ys) (auto simp: suffix_def Cons_eq_append_conv)

lemma distinct_suffixes [intro]: "distinct (suffixes xs)"
by (induction xs) (auto simp: suffix_def)

lemma length_suffixes [simp]: "length (suffixes xs) = Suc (length xs)"
by (induction xs) auto

lemma suffixes_snoc [simp]: "suffixes (xs @ [x]) = [] # map (λys. ys @ [x]) (suffixes xs)"
by (induction xs) auto

lemma suffixes_not_Nil [simp]: "suffixes xs ≠ []"
by (cases xs) auto

lemma hd_suffixes [simp]: "hd (suffixes xs) = []"
by (induction xs) simp_all

lemma last_suffixes [simp]: "last (suffixes xs) = xs"
by (cases xs) simp_all

lemma suffixes_append:
"suffixes (xs @ ys) = suffixes ys @ map (λxs'. xs' @ ys) (tl (suffixes xs))"
proof (induction ys rule: rev_induct)
case Nil
thus ?case by (cases xs rule: rev_cases) auto
next
case (snoc y ys)
show ?case
by (simp only: append.assoc [symmetric] suffixes_snoc snoc.IH) simp
qed

lemma suffixes_eq_snoc:
"suffixes ys = xs @ [x] ⟷
(ys = [] ∧ xs = [] ∨ (∃z zs. ys = z#zs ∧ xs = suffixes zs)) ∧ x = ys"
by (cases ys) auto

lemma suffixes_tailrec [code]:
"suffixes xs = rev (snd (foldl (λ(acc1, acc2) x. (x#acc1, (x#acc1)#acc2)) ([],[[]]) (rev xs)))"
proof -
have "foldl (λ(acc1, acc2) x. (x#acc1, (x#acc1)#acc2)) (ys, ys # zs) (rev xs) =
(xs @ ys, rev (map (λas. as @ ys) (suffixes xs)) @ zs)" for ys zs
proof (induction xs arbitrary: ys zs)
case (Cons x xs ys zs)
from Cons.IH[of ys zs]
show ?case by (simp add: o_def case_prod_unfold)
qed simp_all
from this [of "[]" "[]"] show ?thesis by simp
qed

lemma set_suffixes_eq: "set (suffixes xs) = {ys. suffix ys xs}"
by auto

lemma card_set_suffixes [simp]: "card (set (suffixes xs)) = Suc (length xs)"
by (subst distinct_card) auto

lemma set_suffixes_append:
"set (suffixes (xs @ ys)) = set (suffixes ys) ∪ {xs' @ ys |xs'. xs' ∈ set (suffixes xs)}"
by (subst suffixes_append, cases xs rule: rev_cases) auto

lemma suffixes_conv_prefixes: "suffixes xs = map rev (prefixes (rev xs))"
by (induction xs) auto

lemma prefixes_conv_suffixes: "prefixes xs = map rev (suffixes (rev xs))"
by (induction xs) auto

lemma prefixes_rev: "prefixes (rev xs) = map rev (suffixes xs)"
by (induction xs) auto

lemma suffixes_rev: "suffixes (rev xs) = map rev (prefixes xs)"
by (induction xs) auto

subsection ‹Homeomorphic embedding on lists›

inductive list_emb :: "('a ⇒ 'a ⇒ bool) ⇒ 'a list ⇒ 'a list ⇒ bool"
for P :: "('a ⇒ 'a ⇒ bool)"
where
list_emb_Nil [intro, simp]: "list_emb P [] ys"
| list_emb_Cons [intro] : "list_emb P xs ys ⟹ list_emb P xs (y#ys)"
| list_emb_Cons2 [intro]: "P x y ⟹ list_emb P xs ys ⟹ list_emb P (x#xs) (y#ys)"

lemma list_emb_mono:
assumes "⋀x y. P x y ⟶ Q x y"
shows "list_emb P xs ys ⟶ list_emb Q xs ys"
proof
assume "list_emb P xs ys"
then show "list_emb Q xs ys" by (induct) (auto simp: assms)
qed

lemma list_emb_Nil2 [simp]:
assumes "list_emb P xs []" shows "xs = []"
using assms by (cases rule: list_emb.cases) auto

lemma list_emb_refl:
assumes "⋀x. x ∈ set xs ⟹ P x x"
shows "list_emb P xs xs"
using assms by (induct xs) auto

lemma list_emb_Cons_Nil [simp]: "list_emb P (x#xs) [] = False"
proof -
{ assume "list_emb P (x#xs) []"
from list_emb_Nil2 [OF this] have False by simp
} moreover {
assume False
then have "list_emb P (x#xs) []" by simp
} ultimately show ?thesis by blast
qed

lemma list_emb_append2 [intro]: "list_emb P xs ys ⟹ list_emb P xs (zs @ ys)"
by (induct zs) auto

lemma list_emb_prefix [intro]:
assumes "list_emb P xs ys" shows "list_emb P xs (ys @ zs)"
using assms
by (induct arbitrary: zs) auto

lemma list_emb_ConsD:
assumes "list_emb P (x#xs) ys"
shows "∃us v vs. ys = us @ v # vs ∧ P x v ∧ list_emb P xs vs"
using assms
proof (induct x ≡ "x # xs" ys arbitrary: x xs)
case list_emb_Cons
then show ?case by (metis append_Cons)
next
case (list_emb_Cons2 x y xs ys)
then show ?case by blast
qed

lemma list_emb_appendD:
assumes "list_emb P (xs @ ys) zs"
shows "∃us vs. zs = us @ vs ∧ list_emb P xs us ∧ list_emb P ys vs"
using assms
proof (induction xs arbitrary: ys zs)
case Nil then show ?case by auto
next
case (Cons x xs)
then obtain us v vs where
zs: "zs = us @ v # vs" and p: "P x v" and lh: "list_emb P (xs @ ys) vs"
by (auto dest: list_emb_ConsD)
obtain sk⇩0 :: "'a list ⇒ 'a list ⇒ 'a list" and sk⇩1 :: "'a list ⇒ 'a list ⇒ 'a list" where
sk: "∀x⇩0 x⇩1. ¬ list_emb P (xs @ x⇩0) x⇩1 ∨ sk⇩0 x⇩0 x⇩1 @ sk⇩1 x⇩0 x⇩1 = x⇩1 ∧ list_emb P xs (sk⇩0 x⇩0 x⇩1) ∧ list_emb P x⇩0 (sk⇩1 x⇩0 x⇩1)"
using Cons(1) by (metis (no_types))
hence "∀x⇩2. list_emb P (x # xs) (x⇩2 @ v # sk⇩0 ys vs)" using p lh by auto
thus ?case using lh zs sk by (metis (no_types) append_Cons append_assoc)
qed

lemma list_emb_strict_suffix:
assumes "list_emb P xs ys" and "strict_suffix ys zs"
shows "list_emb P xs zs"
using assms(2) and list_emb_append2 [OF assms(1)] by (auto simp: strict_suffix_def suffix_def)

lemma list_emb_suffix:
assumes "list_emb P xs ys" and "suffix ys zs"
shows "list_emb P xs zs"
using assms and list_emb_strict_suffix
unfolding strict_suffix_reflclp_conv[symmetric] by auto

lemma list_emb_length: "list_emb P xs ys ⟹ length xs ≤ length ys"
by (induct rule: list_emb.induct) auto

lemma list_emb_trans:
assumes "⋀x y z. ⟦x ∈ set xs; y ∈ set ys; z ∈ set zs; P x y; P y z⟧ ⟹ P x z"
shows "⟦list_emb P xs ys; list_emb P ys zs⟧ ⟹ list_emb P xs zs"
proof -
assume "list_emb P xs ys" and "list_emb P ys zs"
then show "list_emb P xs zs" using assms
proof (induction arbitrary: zs)
case list_emb_Nil show ?case by blast
next
case (list_emb_Cons xs ys y)
from list_emb_ConsD [OF ‹list_emb P (y#ys) zs›] obtain us v vs
where zs: "zs = us @ v # vs" and "P⇧=⇧= y v" and "list_emb P ys vs" by blast
then have "list_emb P ys (v#vs)" by blast
then have "list_emb P ys zs" unfolding zs by (rule list_emb_append2)
from list_emb_Cons.IH [OF this] and list_emb_Cons.prems show ?case by auto
next
case (list_emb_Cons2 x y xs ys)
from list_emb_ConsD [OF ‹list_emb P (y#ys) zs›] obtain us v vs
where zs: "zs = us @ v # vs" and "P y v" and "list_emb P ys vs" by blast
with list_emb_Cons2 have "list_emb P xs vs" by auto
moreover have "P x v"
proof -
from zs have "v ∈ set zs" by auto
moreover have "x ∈ set (x#xs)" and "y ∈ set (y#ys)" by simp_all
ultimately show ?thesis
using ‹P x y› and ‹P y v› and list_emb_Cons2
by blast
qed
ultimately have "list_emb P (x#xs) (v#vs)" by blast
then show ?case unfolding zs by (rule list_emb_append2)
qed
qed

lemma list_emb_set:
assumes "list_emb P xs ys" and "x ∈ set xs"
obtains y where "y ∈ set ys" and "P x y"
using assms by (induct) auto

lemma list_emb_Cons_iff1 [simp]:
assumes "P x y"
shows   "list_emb P (x#xs) (y#ys) ⟷ list_emb P xs ys"
using assms by (subst list_emb.simps) (auto dest: list_emb_ConsD)

lemma list_emb_Cons_iff2 [simp]:
assumes "¬P x y"
shows   "list_emb P (x#xs) (y#ys) ⟷ list_emb P (x#xs) ys"
using assms by (subst list_emb.simps) auto

lemma list_emb_code [code]:
"list_emb P [] ys ⟷ True"
"list_emb P (x#xs) [] ⟷ False"
"list_emb P (x#xs) (y#ys) ⟷ (if P x y then list_emb P xs ys else list_emb P (x#xs) ys)"
by simp_all

subsection ‹Subsequences (special case of homeomorphic embedding)›

abbreviation subseq :: "'a list ⇒ 'a list ⇒ bool"
where "subseq xs ys ≡ list_emb (=) xs ys"

definition strict_subseq where "strict_subseq xs ys ⟷ xs ≠ ys ∧ subseq xs ys"

lemma subseq_Cons2: "subseq xs ys ⟹ subseq (x#xs) (x#ys)" by auto

lemma subseq_same_length:
assumes "subseq xs ys" and "length xs = length ys" shows "xs = ys"
using assms by (induct) (auto dest: list_emb_length)

lemma not_subseq_length [simp]: "length ys < length xs ⟹ ¬ subseq xs ys"
by (metis list_emb_length linorder_not_less)

lemma subseq_Cons': "subseq (x#xs) ys ⟹ subseq xs ys"
by (induct xs, simp, blast dest: list_emb_ConsD)

lemma subseq_Cons2':
assumes "subseq (x#xs) (x#ys)" shows "subseq xs ys"
using assms by (cases) (rule subseq_Cons')

lemma subseq_Cons2_neq:
assumes "subseq (x#xs) (y#ys)"
shows "x ≠ y ⟹ subseq (x#xs) ys"
using assms by (cases) auto

lemma subseq_Cons2_iff [simp]:
"subseq (x#xs) (y#ys) = (if x = y then subseq xs ys else subseq (x#xs) ys)"
by simp

lemma subseq_append': "subseq (zs @ xs) (zs @ ys) ⟷ subseq xs ys"
by (induct zs) simp_all

interpretation subseq_order: order subseq strict_subseq
proof
fix xs ys :: "'a list"
{
assume "subseq xs ys" and "subseq ys xs"
thus "xs = ys"
proof (induct)
case list_emb_Nil
from list_emb_Nil2 [OF this] show ?case by simp
next
case list_emb_Cons2
thus ?case by simp
next
case list_emb_Cons
hence False using subseq_Cons' by fastforce
thus ?case ..
qed
}
thus "strict_subseq xs ys ⟷ (subseq xs ys ∧ ¬subseq ys xs)"
by (auto simp: strict_subseq_def)
qed (auto simp: list_emb_refl intro: list_emb_trans)

lemma in_set_subseqs [simp]: "xs ∈ set (subseqs ys) ⟷ subseq xs ys"
proof
assume "xs ∈ set (subseqs ys)"
thus "subseq xs ys"
by (induction ys arbitrary: xs) (auto simp: Let_def)
next
have [simp]: "[] ∈ set (subseqs ys)" for ys :: "'a list"
by (induction ys) (auto simp: Let_def)
assume "subseq xs ys"
thus "xs ∈ set (subseqs ys)"
by (induction xs ys rule: list_emb.induct) (auto simp: Let_def)
qed

lemma set_subseqs_eq: "set (subseqs ys) = {xs. subseq xs ys}"
by auto

lemma subseq_append_le_same_iff: "subseq (xs @ ys) ys ⟷ xs = []"
by (auto dest: list_emb_length)

lemma subseq_singleton_left: "subseq [x] ys ⟷ x ∈ set ys"
by (fastforce dest: list_emb_ConsD split_list_last)

lemma list_emb_append_mono:
"⟦ list_emb P xs xs'; list_emb P ys ys' ⟧ ⟹ list_emb P (xs@ys) (xs'@ys')"
by (induct rule: list_emb.induct) auto

lemma prefix_imp_subseq [intro]: "prefix xs ys ⟹ subseq xs ys"
by (auto simp: prefix_def)

lemma suffix_imp_subseq [intro]: "suffix xs ys ⟹ subseq xs ys"
by (auto simp: suffix_def)

subsection ‹Appending elements›

lemma subseq_append [simp]:
"subseq (xs @ zs) (ys @ zs) ⟷ subseq xs ys" (is "?l = ?r")
proof
{ fix xs' ys' xs ys zs :: "'a list" assume "subseq xs' ys'"
then have "xs' = xs @ zs ∧ ys' = ys @ zs ⟶ subseq xs ys"
proof (induct arbitrary: xs ys zs)
case list_emb_Nil show ?case by simp
next
case (list_emb_Cons xs' ys' x)
{ assume "ys=[]" then have ?case using list_emb_Cons(1) by auto }
moreover
{ fix us assume "ys = x#us"
then have ?case using list_emb_Cons(2) by(simp add: list_emb.list_emb_Cons) }
ultimately show ?case by (auto simp:Cons_eq_append_conv)
next
case (list_emb_Cons2 x y xs' ys')
{ assume "xs=[]" then have ?case using list_emb_Cons2(1) by auto }
moreover
{ fix us vs assume "xs=x#us" "ys=x#vs" then have ?case using list_emb_Cons2 by auto}
moreover
{ fix us assume "xs=x#us" "ys=[]" then have ?case using list_emb_Cons2(2) by bestsimp }
ultimately show ?case using ‹(=) x y› by (auto simp: Cons_eq_append_conv)
qed }
moreover assume ?l
ultimately show ?r by blast
next
assume ?r then show ?l by (metis list_emb_append_mono subseq_order.order_refl)
qed

lemma subseq_append_iff:
"subseq xs (ys @ zs) ⟷ (∃xs1 xs2. xs = xs1 @ xs2 ∧ subseq xs1 ys ∧ subseq xs2 zs)"
(is "?lhs = ?rhs")
proof
assume ?lhs thus ?rhs
proof (induction xs "ys @ zs" arbitrary: ys zs rule: list_emb.induct)
case (list_emb_Cons xs ws y ys zs)
from list_emb_Cons(2)[of "tl ys" zs] and list_emb_Cons(2)[of "[]" "tl zs"] and list_emb_Cons(1,3)
show ?case by (cases ys) auto
next
case (list_emb_Cons2 x y xs ws ys zs)
from list_emb_Cons2(3)[of "tl ys" zs] and list_emb_Cons2(3)[of "[]" "tl zs"]
and list_emb_Cons2(1,2,4)
show ?case by (cases ys) (auto simp: Cons_eq_append_conv)
qed auto
qed (auto intro: list_emb_append_mono)

lemma subseq_appendE [case_names append]:
assumes "subseq xs (ys @ zs)"
obtains xs1 xs2 where "xs = xs1 @ xs2" "subseq xs1 ys" "subseq xs2 zs"
using assms by (subst (asm) subseq_append_iff) auto

lemma subseq_drop_many: "subseq xs ys ⟹ subseq xs (zs @ ys)"
by (induct zs) auto

lemma subseq_rev_drop_many: "subseq xs ys ⟹ subseq xs (ys @ zs)"
by (metis append_Nil2 list_emb_Nil list_emb_append_mono)

subsection ‹Relation to standard list operations›

lemma subseq_map:
assumes "subseq xs ys" shows "subseq (map f xs) (map f ys)"
using assms by (induct) auto

lemma subseq_filter_left [simp]: "subseq (filter P xs) xs"
by (induct xs) auto

lemma subseq_filter [simp]:
assumes "subseq xs ys" shows "subseq (filter P xs) (filter P ys)"
using assms by induct auto

lemma subseq_conv_nths:
"subseq xs ys ⟷ (∃N. xs = nths ys N)" (is "?L = ?R")
proof
assume ?L
then show ?R
proof (induct)
case list_emb_Nil show ?case by (metis nths_empty)
next
case (list_emb_Cons xs ys x)
then obtain N where "xs = nths ys N" by blast
then have "xs = nths (x#ys) (Suc ` N)"
by (clarsimp simp add: nths_Cons inj_image_mem_iff)
then show ?case by blast
next
case (list_emb_Cons2 x y xs ys)
then obtain N where "xs = nths ys N" by blast
then have "x#xs = nths (x#ys) (insert 0 (Suc ` N))"
by (clarsimp simp add: nths_Cons inj_image_mem_iff)
moreover from list_emb_Cons2 have "x = y" by simp
ultimately show ?case by blast
qed
next
assume ?R
then obtain N where "xs = nths ys N" ..
moreover have "subseq (nths ys N) ys"
proof (induct ys arbitrary: N)
case Nil show ?case by simp
next
case Cons then show ?case by (auto simp: nths_Cons)
qed
ultimately show ?L by simp
qed

subsection ‹Contiguous sublists›

subsubsection ‹‹sublist››

definition sublist :: "'a list ⇒ 'a list ⇒ bool" where
"sublist xs ys = (∃ps ss. ys = ps @ xs @ ss)"

definition strict_sublist :: "'a list ⇒ 'a list ⇒ bool" where
"strict_sublist xs ys ⟷ sublist xs ys ∧ xs ≠ ys"

interpretation sublist_order: order sublist strict_sublist
proof
fix xs ys zs :: "'a list"
assume "sublist xs ys" "sublist ys zs"
then obtain xs1 xs2 ys1 ys2 where "ys = xs1 @ xs @ xs2" "zs = ys1 @ ys @ ys2"
by (auto simp: sublist_def)
hence "zs = (ys1 @ xs1) @ xs @ (xs2 @ ys2)" by simp
thus "sublist xs zs" unfolding sublist_def by blast
next
fix xs ys :: "'a list"
{
assume "sublist xs ys" "sublist ys xs"
then obtain as bs cs ds
where xs: "xs = as @ ys @ bs" and ys: "ys = cs @ xs @ ds"
by (auto simp: sublist_def)
have "xs = as @ cs @ xs @ ds @ bs" by (subst xs, subst ys) auto
also have "length … = length as + length cs + length xs + length bs + length ds"
by simp
finally have "as = []" "bs = []" by simp_all
with xs show "xs = ys" by simp
}
thus "strict_sublist xs ys ⟷ (sublist xs ys ∧ ¬sublist ys xs)"
by (auto simp: strict_sublist_def)
qed (auto simp: strict_sublist_def sublist_def intro: exI[of _ "[]"])

lemma sublist_Nil_left [simp, intro]: "sublist [] ys"
by (auto simp: sublist_def)

lemma sublist_Cons_Nil [simp]: "¬sublist (x#xs) []"
by (auto simp: sublist_def)

lemma sublist_Nil_right [simp]: "sublist xs [] ⟷ xs = []"
by (cases xs) auto

lemma sublist_appendI [simp, intro]: "sublist xs (ps @ xs @ ss)"
by (auto simp: sublist_def)

lemma sublist_append_leftI [simp, intro]: "sublist xs (ps @ xs)"
by (auto simp: sublist_def intro: exI[of _ "[]"])

lemma sublist_append_rightI [simp, intro]: "sublist xs (xs @ ss)"
by (auto simp: sublist_def intro: exI[of _ "[]"])

lemma sublist_altdef: "sublist xs ys ⟷ (∃ys'. prefix ys' ys ∧ suffix xs ys')"
proof safe
assume "sublist xs ys"
then obtain ps ss where "ys = ps @ xs @ ss" by (auto simp: sublist_def)
thus "∃ys'. prefix ys' ys ∧ suffix xs ys'"
by (intro exI[of _ "ps @ xs"] conjI suffix_appendI) auto
next
fix ys'
assume "prefix ys' ys" "suffix xs ys'"
thus "sublist xs ys" by (auto simp: prefix_def suffix_def)
qed

lemma sublist_altdef': "sublist xs ys ⟷ (∃ys'. suffix ys' ys ∧ prefix xs ys')"
proof safe
assume "sublist xs ys"
then obtain ps ss where "ys = ps @ xs @ ss" by (auto simp: sublist_def)
thus "∃ys'. suffix ys' ys ∧ prefix xs ys'"
by (intro exI[of _ "xs @ ss"] conjI suffixI) auto
next
fix ys'
assume "suffix ys' ys" "prefix xs ys'"
thus "sublist xs ys" by (auto simp: prefix_def suffix_def)
qed

lemma sublist_Cons_right: "sublist xs (y # ys) ⟷ prefix xs (y # ys) ∨ sublist xs ys"
by (auto simp: sublist_def prefix_def Cons_eq_append_conv)

lemma sublist_code [code]:
"sublist [] ys ⟷ True"
"sublist (x # xs) [] ⟷ False"
"sublist (x # xs) (y # ys) ⟷ prefix (x # xs) (y # ys) ∨ sublist (x # xs) ys"

lemma sublist_append:
"sublist xs (ys @ zs) ⟷
sublist xs ys ∨ sublist xs zs ∨ (∃xs1 xs2. xs = xs1 @ xs2 ∧ suffix xs1 ys ∧ prefix xs2 zs)"
by (auto simp: sublist_altdef prefix_append suffix_append)

lemma map_mono_sublist:
assumes "sublist xs ys"
shows   "sublist (map f xs) (map f ys)"
proof -
from assms obtain xs1 xs2 where ys: "ys = xs1 @ xs @ xs2"
by (auto simp: sublist_def)
have "map f ys = map f xs1 @ map f xs @ map f xs2"
by (auto simp: ys)
thus ?thesis
by (auto simp: sublist_def)
qed

lemma sublist_length_le: "sublist xs ys ⟹ length xs ≤ length ys"

lemma set_mono_sublist: "sublist xs ys ⟹ set xs ⊆ set ys"

lemma prefix_imp_sublist [simp, intro]: "prefix xs ys ⟹ sublist xs ys"
by (auto simp: sublist_def prefix_def intro: exI[of _ "[]"])

lemma suffix_imp_sublist [simp, intro]: "suffix xs ys ⟹ sublist xs ys"
by (auto simp: sublist_def suffix_def intro: exI[of _ "[]"])

lemma sublist_take [simp, intro]: "sublist (take n xs) xs"
by (rule prefix_imp_sublist) (simp_all add: take_is_prefix)

lemma sublist_drop [simp, intro]: "sublist (drop n xs) xs"
by (rule suffix_imp_sublist) (simp_all add: suffix_drop)

lemma sublist_tl [simp, intro]: "sublist (tl xs) xs"
by (rule suffix_imp_sublist) (simp_all add: suffix_drop)

lemma sublist_butlast [simp, intro]: "sublist (butlast xs) xs"
by (rule prefix_imp_sublist) (simp_all add: prefixeq_butlast)

lemma sublist_rev [simp]: "sublist (rev xs) (rev ys) = sublist xs ys"
proof
assume "sublist (rev xs) (rev ys)"
then obtain as bs where "rev ys = as @ rev xs @ bs"
by (auto simp: sublist_def)
also have "rev … = rev bs @ xs @ rev as" by simp
finally show "sublist xs ys" by simp
next
assume "sublist xs ys"
then obtain as bs where "ys = as @ xs @ bs"
by (auto simp: sublist_def)
also have "rev … = rev bs @ rev xs @ rev as" by simp
finally show "sublist (rev xs) (rev ys)" by simp
qed

lemma sublist_rev_left: "sublist (rev xs) ys = sublist xs (rev ys)"
by (subst sublist_rev [symmetric]) (simp only: rev_rev_ident)

lemma sublist_rev_right: "sublist xs (rev ys) = sublist (rev xs) ys"
by (subst sublist_rev [symmetric]) (simp only: rev_rev_ident)

lemma snoc_sublist_snoc:
"sublist (xs @ [x]) (ys @ [y]) ⟷
(x = y ∧ suffix xs ys ∨ sublist (xs @ [x]) ys) "
by (subst (1 2) sublist_rev [symmetric])
(simp del: sublist_rev add: sublist_Cons_right suffix_to_prefix)

lemma sublist_snoc:
"sublist xs (ys @ [y]) ⟷ suffix xs (ys @ [y]) ∨ sublist xs ys"
by (subst (1 2) sublist_rev [symmetric])
(simp del: sublist_rev add: sublist_Cons_right suffix_to_prefix)

lemma sublist_imp_subseq [intro]: "sublist xs ys ⟹ subseq xs ys"
by (auto simp: sublist_def)

lemma sublist_map_rightE:
assumes "sublist xs (map f ys)"
shows   "∃xs'. sublist xs' ys ∧ xs = map f xs'"
proof -
note takedrop = sublist_take sublist_drop
define n where "n = (length ys - length xs)"
from assms obtain xs1 xs2 where xs12: "map f ys = xs1 @ xs @ xs2"
by (auto simp: sublist_def)
define n where "n = length xs1"
have "xs = take (length xs) (drop n (map f ys))"
thus ?thesis
by (intro exI[of _ "take (length xs) (drop n ys)"])
(auto simp: take_map drop_map intro!: takedrop[THEN sublist_order.order.trans])
qed

assumes "sublist xs ys"
proof -
from assms obtain xs1 xs2 where ys: "ys = xs1 @ xs @ xs2"
by (auto simp: sublist_def)
have "suffix (remdups_adj (xs @ xs2)) (remdups_adj (xs1 @ xs @ xs2))"
by (rule suffix_remdups_adj, rule  suffix_appendI) auto
then obtain zs1 where zs1: "remdups_adj (xs1 @ xs @ xs2) = zs1 @ remdups_adj (xs @ xs2)"
by (auto simp: suffix_def)
then obtain zs2 where zs2: "remdups_adj (xs @ xs2) = remdups_adj xs @ zs2"
by (auto simp: prefix_def)
show ?thesis
by (simp add: ys zs1 zs2)
qed

subsubsection ‹‹sublists››

primrec sublists :: "'a list ⇒ 'a list list" where
"sublists [] = [[]]"
| "sublists (x # xs) = sublists xs @ map ((#) x) (prefixes xs)"

lemma in_set_sublists [simp]: "xs ∈ set (sublists ys) ⟷ sublist xs ys"
by (induction ys arbitrary: xs) (auto simp: sublist_Cons_right prefix_Cons)

lemma set_sublists_eq: "set (sublists xs) = {ys. sublist ys xs}"
by auto

lemma length_sublists [simp]: "length (sublists xs) = Suc (length xs * Suc (length xs) div 2)"
by (induction xs) simp_all

subsection ‹Parametricity›

context includes lifting_syntax
begin

private lemma prefix_primrec:
"prefix = rec_list (λxs. True) (λx xs xsa ys.
case ys of [] ⇒ False | y # ys ⇒ x = y ∧ xsa ys)"
proof (intro ext, goal_cases)
case (1 xs ys)
show ?case by (induction xs arbitrary: ys) (auto simp: prefix_Cons split: list.splits)
qed

private lemma sublist_primrec:
"sublist = (λxs ys. rec_list (λxs. xs = []) (λy ys ysa xs. prefix xs (y # ys) ∨ ysa xs) ys xs)"
proof (intro ext, goal_cases)
case (1 xs ys)
show ?case by (induction ys) (auto simp: sublist_Cons_right)
qed

private lemma list_emb_primrec:
"list_emb = (λuu uua uuaa. rec_list (λP xs. List.null xs) (λy ys ysa P xs. case xs of [] ⇒ True
| x # xs ⇒ if P x y then ysa P xs else ysa P (x # xs)) uuaa uu uua)"
proof (intro ext, goal_cases)
case (1 P xs ys)
show ?case
by (induction ys arbitrary: xs)
(auto simp: list_emb_code List.null_def split: list.splits)
qed

lemma prefix_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 A ===> (=)) prefix prefix"
unfolding prefix_primrec by transfer_prover

lemma suffix_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 A ===> (=)) suffix suffix"
unfolding suffix_to_prefix [abs_def] by transfer_prover

lemma sublist_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 A ===> (=)) sublist sublist"
unfolding sublist_primrec by transfer_prover

lemma parallel_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 A ===> (=)) parallel parallel"
unfolding parallel_def by transfer_prover

lemma list_emb_transfer [transfer_rule]:
"((A ===> A ===> (=)) ===> list_all2 A ===> list_all2 A ===> (=)) list_emb list_emb"
unfolding list_emb_primrec by transfer_prover

lemma strict_prefix_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 A ===> (=)) strict_prefix strict_prefix"
unfolding strict_prefix_def by transfer_prover

lemma strict_suffix_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 A ===> (=)) strict_suffix strict_suffix"
unfolding strict_suffix_def by transfer_prover

lemma strict_subseq_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 A ===> (=)) strict_subseq strict_subseq"
unfolding strict_subseq_def by transfer_prover

lemma strict_sublist_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 A ===> (=)) strict_sublist strict_sublist"
unfolding strict_sublist_def by transfer_prover

lemma prefixes_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 (list_all2 A)) prefixes prefixes"
unfolding prefixes_def by transfer_prover

lemma suffixes_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 (list_all2 A)) suffixes suffixes"
unfolding suffixes_def by transfer_prover

lemma sublists_transfer [transfer_rule]:
assumes [transfer_rule]: "bi_unique A"
shows   "(list_all2 A ===> list_all2 (list_all2 A)) sublists sublists"
unfolding sublists_def by transfer_prover

end

end
```
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