Extensions.cs

public static IEnumerable<T> Descendants<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> DescendBy) {
            foreach (T value in source) {
                yield return value;

                foreach (T child in DescendBy(value).Descendants<T>(DescendBy)) {
                    yield return child;
                }
            }
        }