BitSet getAtomsWithin(String withinWhat, BitSet bs) {
		if (withinWhat.equals("group"))
			return withinGroup(bs);
		if (withinWhat.equals("chain"))
			return withinChain(bs);
		if (withinWhat.equals("molecule"))
			return withinMolecule(bs);
		if (withinWhat.equals("model"))
			return withinModel(bs);
		if (withinWhat.equals("element"))
			return withinElement(bs);
		if (withinWhat.equals("site"))
			return withinSite(bs);
		return null;
	}