def develop(envelope):
    """
    Remove the envelope from a message.

    :param envelope: PokerTHMessage object that envelops a message
    :return: PokerTH message from the envelope
    """
    msg = [v for _, v in envelope.ListFields() if v != envelope.messageType]
    assert len(msg) == 1
    assert msg[0].IsInitialized()
    return msg[0]