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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 watch_source(self, source_id):
 """ Add a souce to the watchlist. """

        source_id = int(source_id)
        r = yield from self._send_cmd(
                "WATCH S[%d] ON" % (source_id, ))
        self._watched_source.add(source_id)
        return r