code: private IRingSet getRingSystemOfAtom(List ringSystems, IAtom ringAtom) {
          IRingSet ringSet = null;
          for (int f = 0; f < ringSystems.size(); f++) {
              ringSet = (IRingSet) ringSystems.get(f);
              if (ringSet.contains(ringAtom)) {
                  return ringSet;
              }
          }
          return null;
      }
docs: Get the ring system of which the given atom is part of
  
  @param ringSystems a List of ring systems to be searched
  @param ringAtom    the ring atom to be search in the ring system.
  @return the ring system the given atom is part of