anchor.diff:  Rb_ : ']' ;
   Rp_ : ')' ;
   Semi_ : ';' ;
  -Times_ : '×' | '*' ;
   Underscore_ : '_' ;
   
   // 2.1
  
positive.code: number : Unsigned_number | Plus_ Unsigned_number | Minus_ Unsigned_number ;
  
  // 2.6.1
  fragment Proper_string : (~('\u231C' | '\u231D'))+ ;
  // rewritten to avoid MLR.
  fragment Open_string : Proper_string? (ULCorner Open_string URCorner)+ ;
  fragment Closed_string : ULCorner Open_string URCorner ;
  String : Closed_string+ | '"' ~'"'* '"' | '`' ~'\''* '\'' ;
  
  // 3
  expression : arithmetic_expression | boolean_expression | designational_expression ;
  
negative.code: // 2.3
  fragment Delimiter : Operator | Separator | Bracket | Declarator | Specificator ;
  fragment Operator : Arithmetic_operator | Relational_operator_f | Logical_operator | Sequential_operator ;
  fragment Arithmetic_operator : Plus_ | Minus_ | Mult_ | Divide_ | Exp_ ;
  fragment Relational_operator_f : Lt_ | Le_ | Eq_ | Ne_ | Gt_ | Ge_ ;
  Relational_operator : Relational_operator_f ;
  fragment Logical_operator : Equiv_ | Includes_ | Or_ | And_ | Not_ ;
  