private static void VisitInternal<T>(T root, Func<T, IEnumerable<T>> next, Action<T> action, ICollection<T> visited = null)
        {
            if(visited == null)
            {
                visited = new HashSet<T>();
            }

            Action<T> step = node =>
                             {
                                action(node);
                                visited.Add(node);
                                VisitInternal(node, next, action, visited);
                             };

            if (!visited.Contains(root))
            {
                step(root);
            }

            foreach (var parent in next(root))
            {
                if (!visited.Contains(parent))
                {
                    step(parent);
                }
            }
        }