'Check if any application service knows this user_id exists.
Args:
user_id(str): The user to query if they exist on any AS.
Returns:
True if this user exists on at least one application service.'

@defer.inlineCallbacks
def query_user_exists(self, user_id):

    user_query_services = (yield self._get_services_for_user(user_id=user_id))
    for user_service in user_query_services:
        is_known_user = (yield self.appservice_api.query_user(user_service, user_id))
        if is_known_user:
            defer.returnValue(True)
    defer.returnValue(False)
