anchor.diff:  //     var void
   //     while with
   //     yield
  +KEYWORD
  +: AWAIT
  +| BREAK
  +| CASE | CATCH | CLASS | CONST | CONTINUE
  +| DEBUGGER | DEFAULT | DELETE | DO
  +| ELSE | EXPORT | EXTENDS
  +| FINALLY | FOR | FUNCTION
  +| IF | IMPORT | IN | INSTANCEOF
  +| NEW
  +| RETURN
  +| SUPER | SWITCH
  +| THIS | THROW | TRY | TYPEOF
  +| VAR | VOID
  +| WHILE | WITH
  +| YIELD
  +;
  +
   AWAIT:      'await' ;
   BREAK:      'break' ;
   CASE:       'case' ;
  
positive.code: | [\uDB40][\uDD00-\uDDEF]
  ;
  
  // ref. Section 11.6.2.1
  // Keyword::
  //     await
  
negative.code: WITH:       'with' ;
  YIELD:      'yield' ;
  
  // ref. Section 11.6.2.2
  // FutureReservedWord::
  //     enum
  FUTURE_RESERVED_WORD
  : ENUM
  ;
  
  ENUM: 'enum' ;
  
  // ref. Section 11.8.1
  // NullLiteral::
  //     null
  NULL_LITERAL
  : 'null'
  ;
  
  // ref. Section 11.8.2
  // BooleanLiteral::
  //     true
  //     false
  BOOLEAN_LITERAL
  : 'true'
  | 'false'
  ;
  
  // ref. section 11.8.4
  // UnicodeEscapeSequence::
  //     'u' Hex4Digits
  