anchor.diff:  //    UnicodeEscapeSequence
   fragment
   EscapeSequence
  -: CharacterEscapeSequence
  -| '0'
  -| HexEscapeSequence
  -| UnicodeEscapeSequence
  +: (CharacterEscapeSequence|'0'|HexEscapeSequence|UnicodeEscapeSequence)
   ;
   
   // CharacterEscapeSequence::
  
positive.code: //    " DoubleStringCharacters[opt] "
  //    ' SingleStringCharacters[opt] '
  StringLiteral
  : '"' DoubleStringCharacters? '"'
  | '\'' SingleStringCharacters? '\''
  ;
  
  // DoubleStringCharacters::
  
negative.code: //     u
  fragment
  EscapeCharacter
  : (SingleEscapeCharacter|DecimalDigit|[xu])
  ;
  
  // HexEscapeSequence::
  