public void FirstLast()
{
XFastTrie<string> trie = new XFastTrie<string>();
Assert.IsNull(trie.First());
Assert.IsNull(trie.Last());
trie.Add(100, "100");
Assert.AreEqual(new KVP(100, "100"), trie.First());
Assert.AreEqual(new KVP(100, "100"), trie.Last());
trie.Add(200, "200");
Assert.AreEqual(new KVP(100, "100"), trie.First());
Assert.AreEqual(new KVP(200, "200"), trie.Last());
}