def METHOD_NAME(self):
    steps = []
    def _append(v):
        steps.append(v)
        dfd = defer.Deferred()
        reactor.callLater(0, dfd.callback, steps)
        return dfd
    dfd = mustbe_deferred(_append, 1)
    dfd.addCallback(self.assertEqual, [1, 2])  # it is [1] with maybeDeferred
    steps.append(2)  # add another value, that should be caught by assertEqual
    return dfd