Interesting question! Actually, at the moment we don’t :).

We did some testing to see how big this issue is and it turned out that it's not dramatic. The main difficulty with graveyard purging is that you need to keep the undo in mind. Elements in the graveyard are used for undo, so garbage collecting will have to be strictly connected with limiting undo steps.

Add to that collaborative changes and it becomes a tougher problem. Therefore, we'd probably start from what seems to be the safest option – trimming the history stack and implementing a garbage collector that goes through all the possible reference points.