docs: public void add(DBIDRef cur, double distance, DBIDRef par, DBIDRef prototype) {
      if(mergecount == 0) {
        prototypes = DataStoreUtil.makeDBIDStorage(ids, DataStoreFactory.HINT_DB | DataStoreFactory.HINT_HOT | DataStoreFactory.HINT_STATIC);
      }
      assert prototypes != null;
      parent.putDBID(cur, par);
      prototypes.putDBID(cur, prototype);
      double olddist = parentDistance.putDouble(cur, distance);
      assert (olddist == Double.POSITIVE_INFINITY) : "Object was already linked!";
      order.putInt(cur, mergecount);
      ++mergecount;
    }