If the server sent a CertificateRequest, we send a Certificate message.
        If no certificate is available, an empty Certificate message is sent:
        - this is a SHOULD in RFC 4346 (Section 7.4.6)
        - this is a MUST in RFC 5246 (Section 7.4.6)

        XXX We may want to add a complete chain.

def should_add_ClientCertificate(self):
        """
        If the server sent a CertificateRequest, we send a Certificate message.
        If no certificate is available, an empty Certificate message is sent:
        - this is a SHOULD in RFC 4346 (Section 7.4.6)
        - this is a MUST in RFC 5246 (Section 7.4.6)

        XXX We may want to add a complete chain.
        """
        hs_msg = [type(m) for m in self.cur_session.handshake_messages_parsed]
        if TLSCertificateRequest not in hs_msg:
            return
        certs = []
        if self.mycert:
            certs = [self.mycert]
        self.add_msg(TLSCertificate(certs=certs))
        raise self.ADDED_CLIENTCERTIFICATE()