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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 append(self, future):
 """Append an object to the linked list. Args: future (PlasmaObjectFuture):
 A PlasmaObjectFuture instance. """

        future.prev = self.tail
        if self.tail is None:
            assert self.head is None
            self.head = future
        else:
            self.tail.next = future
        self.tail = future
        # Once done, it will be removed from the list.
        future.add_done_callback(self.remove)