simplify method

@override Expression simplify()

Possible simplifications:

1. -(-a) = a
2. -0 = 0

Implementation

@override
Expression simplify() {
  final Expression simplifiedOp = exp.simplify();

  // double minus
  if (simplifiedOp is UnaryMinus) {
    return simplifiedOp.exp;
  }

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

  // nothing to do..
  return UnaryMinus(simplifiedOp);
}