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