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" }));
}