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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 digraph_walker_backwards(graph, element, call_back):
 """Visits every element guaranteeing that the previous elements have been visited before"""

    call_back(graph, element)
    for predecessor in graph.predecessors(element):
        call_back(graph, predecessor)
    for predecessor in graph.predecessors(element):
        digraph_walker_backwards(graph, predecessor, call_back)