docs: public void setExpanded(SpatialEntry entry1, SpatialEntry entry2) {
      IntSet exp1 = expanded.get(getPageID(entry1));
      if(exp1 == null) {
        exp1 = new IntOpenHashSet();
        expanded.put(getPageID(entry1), exp1);
      }
      exp1.add(getPageID(entry2));
    }