code: @Override
      public boolean isEmpty() {
          if (setOfMolecules != null && !setOfMolecules.isEmpty()) return false;
          if (setOfReactions != null && !setOfReactions.isEmpty()) return false;
          if (ringSet != null && !ringSet.isEmpty()) return false;
          if (crystal != null && !crystal.isEmpty()) return false;
          return true;
      }
docs: {@inheritDoc}