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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 stop():
 """Stop the server, invalidating any viewer URLs. This allows any previously-referenced data arrays to be garbage collected if there are no other references to them. """

    global global_server
    if global_server is not None:
        ioloop = global_server.ioloop
        def stop_ioloop():
            ioloop.stop()
            ioloop.close()
        global_server.ioloop.add_callback(stop_ioloop)
        global_server = None