def METHOD_NAME(sess, directory, fn_prefix):
    ''' save files related to session's graph into directory.
        - fn_prefix.pb : binary protocol buffer file
        - fn_prefix.pbtxt : text format of protocol buffer file
        - fn_prefix.ckpt.* : checkpoing files contains values of variables
        returns (path of pb file, path of pbtxt file, path of ckpt files)
    '''
    tf.train.write_graph(sess.graph_def, directory, fn_prefix + '.pb', as_text=False)
    tf.train.write_graph(sess.graph_def, directory, fn_prefix + '.pbtxt', as_text=True)
    # save a checkpoint file, which will store the above assignment
    saver = tf.train.Saver()
    saver.save(sess, os.path.join(directory, 'checkoiint', fn_prefix + '.ckpt'))
    return (os.path.join(directory, fn_prefix + '.pb'),
            os.path.join(directory, fn_prefix + '.pbtxt'),
            os.path.join(directory, 'checkoiint', fn_prefix + '.ckpt'))