simplify method

@override Expression simplify()

Possible simplifications:

1. a % -b = a % b
2. 0 % a = 0

Implementation

@override
Expression simplify() {
  final Expression firstOp = first.simplify();
  Expression secondOp = second.simplify();

  if (_isNumber(firstOp, 0)) {
    return firstOp; // = 0
  }

  if (secondOp is UnaryMinus) {
    secondOp = (secondOp).exp;
  }

  return Modulo(firstOp, secondOp);
}