code: public IntSet getExpanded(SpatialEntry entry) {
      IntSet exp = expanded.get(getPageID(entry));
      return (exp != null) ? exp : IntSets.EMPTY_SET;
    }
docs: Returns the nodes which are already expanded with the specified node.
  
  @param entry the id of the node for which the expansions should be returned
  @return the nodes which are already expanded with the specified node