code: protected void add(IIsotope isotope) {
          Integer atomicNum = isotope.getAtomicNumber();
          assert atomicNum != null;
          List<IIsotope> isotopesForElement = isotopes[atomicNum];
          if (isotopesForElement == null) {
              isotopesForElement = new ArrayList<>();
              isotopes[atomicNum] = isotopesForElement;
          }
          isotopesForElement.add(isotope);
      }
docs: Protected methods only to be used by classes extending this class to add
  an IIsotope.