code: public String eatStringConstant() {
  		if (!matchStringConstant())
  			throw new BadSyntaxException();
  		/*
  		 * The input string constant is a quoted string token likes 'str', and
  		 * its token type (ttype) is the quote character. So the string
  		 * constants are not converted to lower case.
  		 */
  		String s = tok.sval;
  		nextToken();
  		return s;
  	}
docs: Throws an exception if the current token is not a string. Otherwise,
  returns that string and moves to the next token.
  
  @return the string value of the current token