public void Reset()
{
RPN test = new RPN("x^2").Compute();
PostFix math = new PostFix(test);
Assert.AreEqual(4, math.SetVariable("x", 2).Compute());
math.Reset();
Assert.AreEqual(9, math.SetVariable("x", 3).Compute());
math.Reset();
Assert.AreEqual(16, math.SetVariable("x", 4).Compute());
}