docs: private WritableIntegerDataStore computeSubtreeSizes(DBIDs order) {
      WritableIntegerDataStore siz = DataStoreUtil.makeIntegerStorage(ids, DataStoreFactory.HINT_HOT | DataStoreFactory.HINT_TEMP, 1);
      DBIDVar v1 = DBIDUtil.newVar();
      for(DBIDIter it = order.iter(); it.valid(); it.advance()) {
        if(DBIDUtil.equal(it, parent.assignVar(it, v1))) {
          continue;
        }
        siz.increment(v1, siz.intValue(it));
      }
      return siz;
    }