code: @Override
      public int getIsotopeCount(IIsotope isotope) {
          return !contains(isotope) ? 0 : isotopes.get(getIsotope(isotope));
      }
docs: Checks a set of Nodes for the occurrence of the isotope in the
  IMolecularFormula from a particular isotope. It returns 0 if the does not exist.
  
  @param   isotope          The IIsotope to look for
  @return                   The occurrence of this isotope in this IMolecularFormula
  @see                      #getIsotopeCount()