code: @Override
      public boolean isEmpty() {
          boolean res = true;
          if (setOfMolecules != null && !setOfMolecules.isEmpty()) res = false;
          if (setOfReactions != null && !setOfReactions.isEmpty()) res = false;
          if (ringSet != null && !ringSet.isEmpty()) res = false;
          if (crystal != null && !crystal.isEmpty()) res = false;
          logger.debug("Checking if chemModel is empty: ", res);
          return res;
      }
docs: {@inheritDoc}