docs: def _verify(self, connection, cert, errorNumber, errorDepth, returnCode):
          """Verify a certificate.
  
          """
          try:
              user = userForCert(self.store, cert)
          except ItemNotFound:
              log.msg("Connection attempt by {0!r}, but no user with that "
                      "e-mail address was found, cert digest was {1}"
                      .format(emailForCert(cert), cert.digest("sha512")))
              return False
  
          digest = cert.digest("sha512")
          if user.digest is None:
              user.digest = digest
              log.msg("First connection by {0!r}, stored digest: {1}"
                      .format(user.email, digest))
              return True
          elif user.digest == digest:
              log.msg("Successful connection by {0!r}".format(user.email))
              return True
          else:
              log.msg("Failed connection by {0!r}; cert digest was {1}, "
                      "expecting {2}".format(user.email, digest, user.digest))
              return False