code: @Override
      public void add(IRingSet ringSet) {
          for (int f = 0; f < ringSet.getAtomContainerCount(); f++) {
              if (!contains(ringSet.getAtomContainer(f))) {
                  addAtomContainer(ringSet.getAtomContainer(f));
              }
          }
      }
docs: Adds all rings of another RingSet if they are not already part of this ring set.
  
  If you want to add a single ring to the set use {@link #addAtomContainer(org.openscience.cdk.interfaces.IAtomContainer)}
  
  @param   ringSet  the ring set to be united with this one.