anchor.diff:  //    " DoubleStringCharacters[opt] "
   //    ' SingleStringCharacters[opt] '
   StringLiteral
  -: '"' DoubleStringCharacters? '"'
  -| '\'' SingleStringCharacters? '\''
  +: ('"' DoubleStringCharacters? '"'|'\'' SingleStringCharacters? '\'')
   ;
   
   // DoubleStringCharacters::
  
positive.code: //    LineContinuation
  fragment
  SingleStringCharacter
  : ~['\\\n\r\u2028\u2029]
  | '\\' EscapeSequence
  | LineContinuation
  ;
  
  // LineContinuation::
  
negative.code: //    UnicodeEscapeSequence
  fragment
  EscapeSequence
  : (CharacterEscapeSequence|'0'|HexEscapeSequence|UnicodeEscapeSequence)
  ;
  
  // CharacterEscapeSequence::
  