code: @Override
      public IMolecularFormula add(IMolecularFormula formula) {
          for (IIsotope newIsotope : formula.isotopes()) {
              addIsotope(newIsotope, formula.getIsotopeCount(newIsotope));
          }
          if (formula.getCharge() != null) {
              if (charge != null)
                  charge += formula.getCharge();
              else
                  charge = formula.getCharge();
          }
          return this;
      }
docs: Adds an molecularFormula to this MolecularFormula.
  
  @param  formula  The molecularFormula to be added to this chemObject
  @return          The IMolecularFormula