code: public Graph<T> next() {
          if (nextSubgraphs.isEmpty()) 
              throw new NoSuchElementException();
          Graph<T> next = nextSubgraphs.poll();
          
          // If we've exhausted the current set of subgraphs, queue up more of
          // them, generated from the remaining vertices
          if (nextSubgraphs.isEmpty())
              advance();
          return next;
      }
docs: Returns the next subgraph from the backing graph.