code: public int index(G g) {
          for (Map.Entry<G,Integer> e : graphIndices.entrySet()) {
              if (isoTest.areIsomorphic(g, e.getKey())) 
                  return e.getValue();
          }
          // No index assigned, so create a new one
          int index = graphIndices.size();
          graphIndices.put(g, index);
          return index;
      }
docs: {@inheritDoc}