code: @Override
      public boolean contains(IAtom atom) {
          for (int i = 0; i < getAtomContainerCount(); i++) {
              if (getAtomContainer(i).contains(atom)) {
                  return true;
              }
          }
          return false;
      }
docs: True, if at least one of the rings in the ringset contains
  the given atom.
  
  @param  atom Atom to check
  @return      true, if the ringset contains the atom