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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def url(self):
 """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 """

        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