public void Descendents()
 {
     var tree = new TreeNode("a",
         new TreeNode("b"),
         new TreeNode("c",
             new TreeNode("d"),
             new TreeNode("e"),
         new TreeNode("f")));
     Assert.IsTrue(
         tree.Descendents(n => n.Children)
             .Select(n => n.Label)
             .SequenceEqual(new[] { "b", "c", "d", "e", "f" }));
 }