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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 is_registered(self, event_type, callback, details_filter=None):
 """Check if a callback is registered. :param event_type: event type callback was registered to :param callback: callback that was used during registration :param details_filter: details filter that was used during registration :returns: if the callback is registered :rtype: boolean """

        listeners = self._topics.get(event_type, [])
        for listener in listeners:
            if listener.is_equivalent(callback, details_filter=details_filter):
                return True
        return False