anchor.diff:    | RPAREN
     | LPAREN
     | QUESTION
  +  | MINUS
     | PLUS
     | STAR
     | BAR
  
positive.code:     | TOKEN_RPAREN
      | TOKEN_LPAREN
      | TOKEN_QUESTION
      | TOKEN_PLUS
      | TOKEN_STAR
      | TOKEN_BAR
  
negative.code:     | NONTERM_seq
      | NONTERM_seq_elem_list
      | NONTERM_seq_elem
      | NONTERM_omit_opt
      | NONTERM_predicate_opt
      | NONTERM_bound
      | NONTERM_patt
  