BitSet withinSite(BitSet bs) {
		if (frame == null)
			return null;
		// Logger.debug("withinGroup");
		BitSet bsResult = new BitSet();
		BitSet bsThis = new BitSet();
		for (int i = getAtomCount(); --i >= 0;)
			if (bs.get(i))
				bsThis.set(frame.getAtomAt(i).atomSite);
		for (int i = getAtomCount(); --i >= 0;)
			if (bsThis.get(frame.getAtomAt(i).atomSite))
				bsResult.set(i);
		return bsResult;
	}