code: private IRingSet recoverRingSystem(IAtomContainer mol) {
          IRingSet ringSet = mol.getBuilder().newInstance(IRingSet.class);
          for (Integer[] bondNumbers : listOfRings) {
              IRing ring = mol.getBuilder().newInstance(IRing.class, bondNumbers.length);
              for (int bondNumber : bondNumbers) {
                  IBond bond = mol.getBond(bondNumber);
                  ring.addBond(bond);
                  if (!ring.contains(bond.getBegin())) ring.addAtom(bond.getBegin());
                  if (!ring.contains(bond.getEnd())) ring.addAtom(bond.getEnd());
              }
              ringSet.addAtomContainer(ring);
          }
          return ringSet;
      }
docs: Recovers a RingSet corresponding to a AtomContainer that has been
  stored by storeRingSystem().
  
  @param mol      The IAtomContainer for which to recover the IRingSet.