synchronized (mutex) {
        return delegate().removeLastOccurrence(o);
      }
    }

    @Override
    public void push(E e) {
      synchronized (mutex) {
        delegate().push(e);
      }
    }

    @Override
    public E pop() {
      synchronized (mutex) {
        return delegate().pop();
      }
    }

    @Override

      // afterDone() should be generally fast and only used for cleanup work... but in theory can
      // also be recursive and create StackOverflowErrors
      future.afterDone();
      // push the current set of listeners onto next
      next = future.clearListeners(next);
      future = null;
      while (next != null) {
        Listener curr = next;
        next = next.next;
        /*