code: def url(self):
          
          end_url = ("/accounts/{account_id}/alerts/{alert_id}/mentions/"
                    "{mention_id}/children?")
  
          def without_keys(d, keys):
              return {x: d[x] for x in d if x not in keys}
  
          keys = {"access_token", "account_id", "alert_id"}
          parameters = without_keys(self.params, keys)
  
          for key, value in list(parameters.items()):
              if value != '':
                  end_url += '&' + key + '={' + key + '}'
  
          end_url = end_url.format(**self.params)
          return self._base_url + end_url
docs: The concatenation of the `base_url` and `end_url` that make up the
          resultant url.
  
          :return: the `base_url` and the `end_url`.
          :rtype: str