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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 subscribe(self,topic,callback=None,options=None):
 """ Subscribe to a uri for events from a publisher """

        full_topic = self.get_full_uri(topic)
        result = self.send_and_await_response(SUBSCRIBE(
                                    options=options or {},
                                    topic=full_topic
                                ))
        if result == WAMP_SUBSCRIBED:
            if not callback:
                callback = lambda a: None
            self._subscriptions[result.subscription_id] = [topic,callback]