code: @Override public LabeledGraph<L,E> copy(Set<Integer> vertices) {
          Graph<E> g = super.copy(vertices);
          // Create a copy of the labels.  
          // NOTE: this includes labels for vertices that may not be present in
          //       the new graph.  Not sure if it's the correct behavior yet.
          Indexer<L> labels = new ObjectIndexer<L>(vertexLabels);
          return new LabeledGraph<L,E>(g, labels);
      }
docs: {@inheritDoc}