anchor.diff:  // <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_list : tff_variable (',' tff_variable)*;
   tff_variable : tff_typed_variable | variable;
   tff_typed_variable: variable ':' tff_atomic_type;
   tff_unary_formula: unary_connective tff_unitary_formula | fol_infix_unary;
  
positive.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 ',' fof_formula_tuple_list;
  
  
  // <fol_infix_unary>      ::= <term> <infix_inequality> <term>
  
negative.code: 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_unary_formula: unary_connective fof_unitary_formula | fol_infix_unary;
  
  //  <fof_sequent>          ::= <fof_formula_tuple> <gentzen_arrow> 
  