anchor.diff:  tff_let : '$let_tf(' tff_let_term_defns ',' tff_formula ')'
           | '$let_ff(' tff_let_formula_defns ',' tff_formula ')';
   tff_let_term_defns : tff_let_term_defn | '[' tff_let_term_list ']';
  -tff_let_term_list : tff_let_term_defn | tff_let_term_defn ',' tff_let_term_list;
  -tff_let_term_defn : '!' '[' tff_variable_list ']' ':' tff_let_term_defn
  +tff_let_term_list : tff_let_term_defn (',' tff_let_term_defn)*;
  +tff_let_term_defn : Forall '[' tff_variable_list ']' ':' tff_let_term_defn
                     | tff_let_term_binding;
  -tff_let_term_binding : plain_term '=' term | '(' tff_let_term_binding ')';
  +tff_let_term_binding : plain_term Infix_equality term | '(' tff_let_term_binding ')';
   tff_let_formula_defns : tff_let_formula_defn | '[' tff_let_formula_list ']';
  -tff_let_formula_list: tff_let_formula_defn | tff_let_formula_defn ',' tff_let_formula_list;
  -tff_let_formula_defn: '!' '[' tff_variable_list ']' ':' tff_let_formula_defn
  +tff_let_formula_list: tff_let_formula_defn (',' tff_let_formula_defn)*;
  +tff_let_formula_defn: Forall '[' tff_variable_list ']' ':' tff_let_formula_defn
                       | tff_let_formula_binding;
  -tff_let_formula_binding: plain_atomic_formula '<=>' tff_unitary_formula
  +tff_let_formula_binding: plain_atomic_formula Iff tff_unitary_formula
                          | '(' tff_let_formula_binding ')';
                          
   // <tff_sequent>          ::= <tff_formula_tuple> <gentzen_arrow> 
  
positive.code: // <tff_conditional>      ::= $ite_f(<tff_logic_formula>,<tff_logic_formula>,
  //                            <tff_logic_formula>)
  tff_quantified_formula : fol_quantifier '[' tff_variable_list ']' ':' tff_unitary_formula;
  tff_variable_list : tff_variable | tff_variable ',' tff_variable_list;
  tff_variable : tff_typed_variable | variable;
  tff_typed_variable: variable ':' tff_atomic_type;
  tff_unary_formula: unary_connective tff_unitary_formula | fol_infix_unary;
  
negative.code: //                             <fof_logic_formula>,<fof_formula_tuple_list>
  fof_sequent : fof_formula_tuple '>>' fof_formula_tuple | '(' fof_sequent ')';
  fof_formula_tuple : '[]' | '[' fof_formula_tuple_list ']';
  fof_formula_tuple_list: fof_logic_formula (',' fof_logic_formula)*;
  
  
  // <fol_infix_unary>      ::= <term> <infix_inequality> <term>
  