public void TestAddBefore()
{
HElement parent = new HElement("parent");
HNode n1 = new HElement("test-1");
parent.Add(n1);
Assert.Equal(1, parent.Nodes().Count());
HNode n2 = new HElement("test-2");
n1.AddBefore(n2);
Assert.Equal(2, parent.Nodes().Count());
n1.AddBefore("1");
Assert.Equal(3, parent.Nodes().Count());
n2.AddBefore("2");
Assert.Equal(4, parent.Nodes().Count());
var nodes = parent.Nodes().ToArray();
Assert.Equal(4, nodes.Length);
Assert.IsType <HText>(nodes[0]);
Assert.IsType <HElement>(nodes[1]);
Assert.IsType <HText>(nodes[2]);
Assert.IsType <HElement>(nodes[3]);
n1 = new HElement("test-3");
var ioe = Assert.Throws <InvalidOperationException>(() => n1.AddBefore(null));
Assert.Equal("No parent found.", ioe.Message);
}