code: @Override
      public double validate(IMolecularFormula formula) throws CDKException {
          logger.info("Start validation of ", formula);
  
          double mass = MolecularFormulaManipulator.getTotalMassNumber(formula);
          if (mass == 0) return 0.0;
  
          int numberN = MolecularFormulaManipulator.getElementCount(formula,
                  formula.getBuilder().newInstance(IElement.class, "N"));
          numberN += getOthers(formula);
  
          if (formula.getCharge() == null || formula.getCharge() == 0 || !isOdd(Math.abs(formula.getCharge()))) {
              if (isOdd(mass) && isOdd(numberN)) {
                  return 1.0;
              } else if (!isOdd(mass) && (numberN == 0 || !isOdd(numberN))) {
                  return 1.0;
              } else
                  return 0.0;
          } else {
              if (!isOdd(mass) && isOdd(numberN)) {
                  return 1.0;
              } else if (isOdd(mass) && (numberN == 0 || !isOdd(numberN))) {
                  return 1.0;
              } else
                  return 0.0;
          }
      }
docs: Validate the nitrogen rule of this IMolecularFormula.
  
  @param formula   Parameter is the IMolecularFormula
  @return          A double value meaning 1.0 True, 0.0 False