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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 enquire_session(self, session=None):
 """ Verifies and returns relevant TensorFlow session. If non-None session were passed, session is checked for graph compliance and returned back. Otherwise, default TensorFlow session is returned. When TensorFlow default session is not set up, GPflow session's manager creates or uses existing one for returning. :param session: TensorFlow session or None. Default value is None. :return: TensorFlow session. :raises GPflowError: Session's graph is not compilable with node's graph. """

        if session is None:
            session = tf.get_default_session()
            if session is None:
                session = session_manager.get_default_session()
        self.is_built_coherence(session.graph)
        return session