public void TestSub(double a, double b, double res)
{
IRPNCalculator c = new RPNCalculator();
Assert.True(c.Size() == 0);
c.Push(a);
c.Push(b);
Assert.True(c.Size() == 2 && c.Top() == 5);
c.Subtract();
Assert.True(c.Size() == 1 && c.Top() == res);
}