anchor.diff:  fof_unitary_formula : fof_quantified_formula | fof_unary_formula | atomic_formula
                       | '(' fof_logic_formula ')';
   fof_quantified_formula : fol_quantifier '[' fof_variable_list ']' ':' fof_unitary_formula;
  -fof_variable_list : variable | variable ',' fof_variable_list;
  +fof_variable_list : variable (',' variable)*;
   fof_unary_formula: unary_connective fof_unitary_formula | fol_infix_unary;
   
   //  <fof_sequent>          ::= <fof_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: // <thf_unary_connective> ::= <unary_connective> | <th1_unary_connective>
  // <th1_unary_connective> ::= !! | ?? | @@+ | @@- | @=
  thf_quantifier : fol_quantifier | th0_quantifier | th1_quantifier;
  th0_quantifier: Lambda | Choice | Description;
  th1_quantifier: TyForall | TyExists;
  thf_pair_connective : Infix_equality | Infix_inequality | binary_connective | Assignment ;
  thf_unary_connective : unary_connective | th1_unary_connective;
  th1_unary_connective : ForallComb | ExistsComb | ChoiceComb | DescriptionComb | EqComb;
  
  // %----Types for THF and TFF
  // <defined_type>         ::= <atomic_defined_word>
  