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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_conjunctive_graph(store_id=None):
 """ Returns an open conjunctive graph. """

    if not store_id:
        store_id = DEFAULT_STORE

    store = DjangoStore(DEFAULT_STORE)
    graph = ConjunctiveGraph(store=store, identifier=store_id)
    if graph.open(None) != VALID_STORE:
        raise ValueError("The store identified by {0} is not a valid store".format(store_id))
    return graph