public void TraverseSingles()
{
var a = new StringNode("a");
var b = new StringNode("b");
var c = new StringNode("c");
var d = new StringNode("d");
var e = new StringNode("e");
var f = new StringNode("f");
var g = new StringNode("g");
// a - b - c - d - e
// - g - f
a.AddFirst(b);
a.AddLast(g);
b.AddFirst(c);
c.AddFirst(d);
d.AddFirst(e);
d.AddLast(f);
string.Join("", b.DescendantsOfSingle().Select(n => n.Value)).Should()
.Be("cd");
string.Join("", b.DescendantsOfSingleAndSelf().Select(n => n.Value)).Should()
.Be("bcd");
string.Join("", c.DescendantsOfSingle().Select(n => n.Value)).Should()
.Be("d");
string.Join("", c.DescendantsOfSingleAndSelf().Select(n => n.Value)).Should()
.Be("cd");
string.Join("", b.AncestorsWithSingleChild().Select(n => n.Value)).Should()
.Be("");
string.Join("", b.AncestorsWithSingleChildAndSelf().Select(n => n.Value)).Should()
.Be("b");
string.Join("", c.AncestorsWithSingleChild().Select(n => n.Value)).Should()
.Be("b");
string.Join("", c.AncestorsWithSingleChildAndSelf().Select(n => n.Value)).Should()
.Be("cb");
string.Join("", d.AncestorsWithSingleChild().Select(n => n.Value)).Should()
.Be("cb");
string.Join("", d.AncestorsWithSingleChildAndSelf().Select(n => n.Value)).Should()
.Be("dcb");
string.Join("", e.AncestorsWithSingleChild().Select(n => n.Value)).Should()
.Be("");
string.Join("", e.AncestorsWithSingleChildAndSelf().Select(n => n.Value)).Should()
.Be("e");
}