XXX Section 7.4.7.1 of RFC 5246 states that the CertificateVerify
        message is only sent following a client certificate that has signing
        capability (i.e. not those containing fixed DH params).
        We should verify that before adding the message. We should also handle
        the case when the Certificate message was empty.

def should_add_ClientVerify(self):
        """
        XXX Section 7.4.7.1 of RFC 5246 states that the CertificateVerify
        message is only sent following a client certificate that has signing
        capability (i.e. not those containing fixed DH params).
        We should verify that before adding the message. We should also handle
        the case when the Certificate message was empty.
        """
        hs_msg = [type(m) for m in self.cur_session.handshake_messages_parsed]
        if (TLSCertificateRequest not in hs_msg or
            self.mycert is None or
                self.mykey is None):
            return
        self.add_msg(TLSCertificateVerify())
        raise self.ADDED_CERTIFICATEVERIFY()