anchor.diff:  
   Or: '|';
   And: '&';
  -Not: '~';
   Iff : '<=>';
   Impl : '=>';
   If: '<=';
   Niff: '<~>';
   Nor: '~|';
   Nand: '~&';
  +Not: '~';
  +ForallComb: '!!';
  +TyForall: '!>';
  +Infix_inequality : '!=';
  +Infix_equality : '=';
   Forall: '!';
  +ExistsComb: '??';
  +TyExists: '?*';
   Exists: '?';
  +Lambda: '^';
  +ChoiceComb: '@@+';
  +Choice: '@+';
  +DescriptionComb: '@@-';
  +Description: '@-';
  +EqComb: '@=';
   App: '@';
  -Infix_equality : '=';
  -Infix_inequality : '!=';
   Assignment: ':=';
   
   // %----Numbers. Signs are made part of the same token here.
  
positive.code: /*  This is a grammar file for TPTP language Version 6.4.0.7
   *  Use ANTLR on this file to obtain the parser.
   *  The BNF specification of the TPTP language was provided by Geoff Sutcliffe.
   *  The grammar file is created by Alexander Steen with help of Tobias Gleißner.
   *  Date: Dec 2016
   */
  grammar tptp;
  
negative.code: 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_defn : Forall '[' tff_variable_list ']' ':' tff_let_term_defn
                    | 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_defn: Forall '[' tff_variable_list ']' ':' tff_let_formula_defn
                      | tff_let_formula_binding;
  tff_let_formula_binding: plain_atomic_formula Iff tff_unitary_formula
                         | '(' tff_let_formula_binding ')';
                         
  // <tff_sequent>          ::= <tff_formula_tuple> <gentzen_arrow> 
  