GIBZ-GIPE

class Calculator {
  double _lastResult = 0;

  double subtract(double a, [double b]) {
    if (b != null) {
      _lastResult = a - b;
    } else {
      _lastResult -= a;
    }
    return _lastResult;

GIBZ-GIPE

class Program
{
    static void Main(string[] args)
    {
        // 1)
        Compare(5, 10);

        // 2)
        Compare("hallo", "hallo");