docs: public void add(DBIDRef cur, double distance, DBIDRef par) {
      assert prototypes == null;
      parent.putDBID(cur, par);
      double olddist = parentDistance.putDouble(cur, distance);
      assert (olddist == Double.POSITIVE_INFINITY) : "Object was already linked!";
      order.putInt(cur, mergecount);
      ++mergecount;
    }