[Test] public void Test_Add()
 {
     SkipList sl = new SkipList();
     Assert.AreEqual(0, sl.Count);
     sl.Add("1", "bar");
     Assert.AreEqual(1, sl.Count);
     sl.Add("2", "baz");
     Assert.AreEqual(2, sl.Count);
 }
 [Test] public void Test_Get()
 {
     SkipList sl = new SkipList();
     sl.Add("1", "bar");
     Assert.AreEqual("bar", sl["1"]);
 }
 [Test] public void Test_Key_Twice()
 {
     SkipList sl = new SkipList();
     sl.Add("one", "one");
     sl.Add("one", "one");
 }
 [Test] public void Test_Null_Key()
 {
     SkipList sl = new SkipList();
     sl.Add(null, "one");
 }