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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 fetch_by_refresh_token(self, refresh_token):
 """ Find an access token by its refresh token. :param refresh_token: The refresh token that was assigned to an ``AccessToken``. :return: The :class:`oauth2.datatype.AccessToken`. :raises: :class:`oauth2.error.AccessTokenNotFound` """

        if refresh_token not in self.refresh_tokens:
            raise AccessTokenNotFound

        return self.refresh_tokens[refresh_token]