code: @Override
      public Collection<List<IAtom>> findRings(IAtomContainer molecule) {
          if (molecule == null) return null;
          rings.clear();
          PathGraph graph = new PathGraph(molecule);
  
          for (int i = 0; i < molecule.getAtomCount(); i++) {
              List<PathEdge> edges = graph.remove(molecule.getAtom(i));
  
              for (PathEdge edge : edges) {
                  List<IAtom> ring = edge.getAtoms();
                  rings.add(ring);
              }
          }
  
          return rings;
      }
docs: Returns a collection of rings.
  
  @param  molecule
  @return a {@link Collection} of {@link List}s containing one ring each
  @see org.openscience.cdk.smsd.ring.RingFinder#findRings(org.openscience.cdk.interfaces.IAtomContainer)