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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 checkin(self, place_id, sensor=False):
 """Checks in a user to a place. keyword arguments: place_id -- The unique Google identifier for the relevant place. sensor -- Boolean flag denoting if the location came from a device using its location sensor (default False). """

        data = {'placeid': place_id}
        url, checkin_response = _fetch_remote_json(
                GooglePlaces.CHECKIN_API_URL % (str(sensor).lower(),
                        self.api_key), json.dumps(data), use_http_post=True)
        _validate_response(url, checkin_response)