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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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())))