internal static List<T> BreadthFirstExpansion<T>(T root, Func<T,List<T>> getChildren) {
    List<T> list = new List<T>();
    list.Add(root);
    foreach(T entry in list) {
        List<T> children = getChildren(entry);
        if(children!=null) {
            foreach (T child in children)
            {
                if (!list.Contains(child)) list.Add(child);
            }
        }
    }
    return list;
}