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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 _not_forever(self, timeout, d):
 """If the timer fires first, cancel the deferred. If the deferred fires first, cancel the timer."""

        t = self._reactor.callLater(timeout, d.cancel)

        def _done(res):
            if t.active():
                t.cancel()
            return res

        d.addBoth(_done)
        return d