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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 auto_track_url(track):
 """ Automatically sets the bigDataUrl for `track`. Requirements: * the track must be fully connected, such that its root is a Hub object * the root Hub object must have the Hub.url attribute set * the track must have the `source` attribute set """

    hub = track.root(cls=Hub)

    if hub is None:
        raise ValueError(
            "track is not fully connected because the root is %s" % repr(hub))
    if hub.url is None:
        raise ValueError("hub.url is not set")
    if track.source is None:
        raise ValueError("track.source is not set")