code: def verify(self, keys=None):
          
  
          try:
              res = self._verify()
          except AssertionError as err:
              logger.error("Verification error on the response: %s", err)
              raise
          else:
              if res is None:
                  return None
  
          if not isinstance(self.response, samlp.Response):
              return self
  
          if self.parse_assertion(keys):
              return self
          else:
              logger.error("Could not parse the assertion")
              return None
docs: Verify that the assertion is syntactically correct and the
          signature is correct if present.
  
          :param keys: If not the default key file should be used then use one
          of these.