code: public void addNodeToIndexMap(int index, TreeGraphNode node) {
      indexMap.put(Integer.valueOf(index), node);
    }
docs: Store a mapping from an arbitrary integer index to a node in
  this treegraph.  Normally a client shouldn't need to use this,
  as the nodes are automatically indexed by the
  <code>TreeGraph</code> constructor.
  
  @param index the arbitrary integer index
  @param node  the <code>TreeGraphNode</code> to be indexed