def sort_topologically(graph):
    """
    Stackless topological sorting.

    graph = {
        3: [1],
        5: [3],
        4: [2],
        6: [4],
    }

    sort_topologically(graph)
    [[1, 2], [3, 4], [5, 6]]
    """
    levels_by_name = {}
    names_by_level = defaultdict(list)

    def add_level_to_name(name, level):
        levels_by_name[name] = level
        names_by_level[level].append(name)


    def walk_depth_first(name):
        stack = [name]
        while(stack):
            name = stack.pop()
            if name in levels_by_name:
                continue

            if name not in graph or not graph[name]:
                level = 0
                add_level_to_name(name, level)
                continue

            children = graph[name]

            children_not_calculated = [child for child in children if child not in levels_by_name]
            if children_not_calculated:
                stack.append(name)
                stack.extend(children_not_calculated)
                continue

            level = 1 + max(levels_by_name[lname] for lname in children)
            add_level_to_name(name, level)

    for name in graph:
        walk_depth_first(name)

    return list(itertools.takewhile(lambda x: x is not None,
                                    (names_by_level.get(i, None)
                                     for i in itertools.count())))