anchor.diff:          advance();
           if (isExponentSign(peek)) advance();
           if (!isDigit(peek)) error('Invalid exponent', -1);
  +        advance();
           simple = false;
         } else {
           break;
         }
  -      advance();
       }
       String string = input.substring(start, index);
       num value = simple ? int.parse(string) : double.parse(string);
  
positive.code:     String string = one;
      if (peek == code) {
        advance();
        string = two;
      }
      assert(OPERATORS.contains(string));
      return new Token(start, string)..withOp(string);
  
negative.code:     assert(isDigit(peek));
      bool simple = (index == start);
      while (true) {
        advance();
        if (isDigit(peek)) {
          // Do nothing.
        } else if (peek == $PERIOD) {
  