code: private void advance() {
          next = null;
          while (next == null) {
              // If 
              if (ext != null) {
                  next = ext.next();
                  if (next != null)
                      return;
              }
              if (!vertexIter.hasNext())
                  return;
  
              // Otherwise the extension is null or there are no more remaining
              // subgraphs in it, so create a new 
              int nextVertex = vertexIter.nextInt();
              veryVerbose(LOGGER, "Loading next round of subgraphs starting " +
                          "from vertex %d", nextVertex);
              ext = new Extension(nextVertex);
          }
      }
docs: Advances the state of this iterating, updating {@code next} to be the
  next graph to return or {@code null} if no further subgraphs remain.