code: public IRingSet findEssentialRings() {
          if (atomContainer == null) {
              return null;
          }
          IRingSet ringSet = toRingSet(atomContainer, cycleBasis().essentialCycles());
          //		atomContainer.setProperty(CDKConstants.ESSENTIAL_RINGS, ringSet);
          return ringSet;
      }
docs: Finds the Set of Essential Rings.
  These rings are contained in every possible SSSR.
  The returned set is uniquely defined.
  
  @return      a RingSet containing the Essential Rings