// push the current set of listeners onto next
      next = future.clearListeners(next);
      future = null;
      while (next != null) {
        Listener curr = next;
        next = next.next;
        /*
         * requireNonNull is safe because the listener stack never contains TOMBSTONE until after
         * clearListeners.
         */
        Runnable task = requireNonNull(curr.task);

      // push the current set of listeners onto next
      next = future.clearListeners(next);
      future = null;
      while (next != null) {
        Listener curr = next;
        next = next.next;
        /*
         * requireNonNull is safe because the listener stack never contains TOMBSTONE until after
         * clearListeners.
         */
        Runnable task = requireNonNull(curr.task);