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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 topic_coupling(model, threshold=None, **kwargs):
 """ Two papers are coupled if they both contain a shared topic above a ``threshold``. Parameters model : :class:`.LDAModel` threshold : float Default: ``3./model.Z`` kwargs : kwargs Passed on to :func:`.coupling`\. Returns ------- :ref:`networkx.Graph <networkx:graph>` """

    if not threshold:
        threshold = 3./model.Z
    select = lambda f, v, c, dc: v > threshold

    graph = coupling(model.corpus, 'topics', filter=select, **kwargs)
    graph.name = ''
    return graph