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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 get_subgraphs_by_citation(graph):
 """Stratify the graph based on citations. :type graph: pybel.BELGraph :rtype: dict[tuple[str,str],pybel.BELGraph] """

    rv = defaultdict(graph.fresh_copy)

    for u, v, key, data in graph.edges(keys=True, data=True):
        if CITATION not in data:
            continue
        dk = data[CITATION][CITATION_TYPE], data[CITATION][CITATION_REFERENCE]

        rv[dk].add_edge(u, v, key=key, **data)

    cleanup(graph, rv)

    return dict(rv)