public void TestAfterElement()
{
var element1 = new HElement("h1");
var element2 = new HElement("h2");
var container = new HElement("div", element1, element2).AddClass("container");
element1.After(new HElement("p"));
Assert.Equal(new String[] { "h1", "p", "h2" }, container.Elements().Select(e => e.Name));
var ex = Assert.Throws <InvalidOperationException>(() => container.After(new HElement("p")));
Assert.Equal("No parent found.", ex.Message);
}