public void TestPredecessor()
        {
            Assert.AreEqual(tree.Nil, tree.Predecessor(tree.Minimum(tree.Root)));
            Assert.AreSame(tree.Nil, tree.Predecessor(tree.Minimum(tree.Root)));

            Assert.AreEqual(tree.Root.Left.Right, tree.Predecessor(tree.Root));
            Assert.AreSame(tree.Root.Left.Right, tree.Predecessor(tree.Root));

            Assert.AreEqual(tree.Root.Left.Left, tree.Predecessor(tree.Get(0)));
            Assert.AreSame(tree.Root.Left.Left, tree.Predecessor(tree.Get(0)));
        }