If last Operation's value of `done` is true, returns false;
        otherwise, issues OperationsClient.cancel_operation and returns true.

def cancel(self):
        """If last Operation's value of `done` is true, returns false;
        otherwise, issues OperationsClient.cancel_operation and returns true.
        """
        if self.done():
            return False

        self._client.cancel_operation(self._operation.name)
        return True