docs: 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()