public virtual void TestBasicOperations()
        {
            TreeShapedStack <string> tss = new TreeShapedStack <string>();

            NUnit.Framework.Assert.AreEqual(tss.size, 0);
            TreeShapedStack <string> tss1 = tss.Push("1");

            NUnit.Framework.Assert.AreEqual(tss1.size, 1);
            NUnit.Framework.Assert.AreEqual(tss1.Peek(), "1");
            TreeShapedStack <string> tss2 = tss1.Push("2");

            NUnit.Framework.Assert.AreEqual(tss2.size, 2);
            NUnit.Framework.Assert.AreEqual(tss2.Peek(), "2");
            NUnit.Framework.Assert.AreEqual(tss2.previous.Peek(), "1");
            TreeShapedStack <string> tss3 = tss2.Push("3");

            NUnit.Framework.Assert.AreEqual(tss3.size, 3);
            NUnit.Framework.Assert.AreEqual(tss3.Peek(), "3");
            NUnit.Framework.Assert.AreEqual(tss3.previous.Peek(), "2");
            tss3 = tss3.Pop();
            NUnit.Framework.Assert.AreEqual(tss3.Peek(), "2");
            NUnit.Framework.Assert.AreEqual(tss3.previous.Peek(), "1");
            NUnit.Framework.Assert.AreEqual(tss3.Peek(), "2");
            TreeShapedStack <string> tss4 = tss3.Push("4");

            NUnit.Framework.Assert.AreEqual(tss4.Peek(), "4");
            NUnit.Framework.Assert.AreEqual(tss4.Peek(), "4");
            NUnit.Framework.Assert.AreEqual(tss4.previous.Peek(), "2");
            tss4 = tss4.Pop();
            NUnit.Framework.Assert.AreEqual(tss4.Peek(), "2");
            tss4 = tss4.Pop();
            NUnit.Framework.Assert.AreEqual(tss4.Peek(), "1");
            tss4 = tss4.Pop();
            NUnit.Framework.Assert.AreEqual(tss4.size, 0);
        }
        public virtual void TestEquals()
        {
            TreeShapedStack <string> t1 = new TreeShapedStack <string>();

            t1 = t1.Push("foo");
            t1 = t1.Push("bar");
            t1 = t1.Push("bar");
            t1 = t1.Push("diet");
            t1 = t1.Push("coke");
            TreeShapedStack <string> t2 = new TreeShapedStack <string>();

            t2 = t2.Push("foo");
            t2 = t2.Push("bar");
            t2 = t2.Push("bar");
            t2 = t2.Push("diet");
            t2 = t2.Push("coke");
            TreeShapedStack <string> t3 = t2.Pop().Push("pepsi");

            NUnit.Framework.Assert.AreEqual(t1, t2);
            NUnit.Framework.Assert.IsFalse(t1.Pop().Equals(t2));
            NUnit.Framework.Assert.IsFalse(t2.Pop().Equals(t1));
            NUnit.Framework.Assert.IsFalse(t2.Equals(t3));
        }