class MassNumberRule implements ISequenceSubRule<ILigand> {

    private Integer getMassNumber(ILigand ligand) {
        Integer massNumber = ligand.getLigandAtom().getMassNumber();
        if (massNumber != null) return massNumber;
        if (factory == null) return 0;
        return factory.getMajorIsotope(ligand.getLigandAtom().getSymbol()).getMassNumber();
    }

}