public void CloneTree_Test()
        {
            var plotter = new Plotter();

            plotter.ProcessString("^(x,3)");

            var expectedResult = plotter.ProcessTree(3, plotter.Root);

            var copy_tree = Plotter.CloneTree(plotter.Root);

            var result = copy_tree.Calculate(3);

            Assert.AreEqual(expectedResult, result);
        }