BitSet lookupIdentifierValue(String identifier) throws ScriptException {
		// all variables and possible residue names for PDB
		// or atom names for non-pdb atoms are processed here.

		// priority is given to a defined variable.

		BitSet bs = lookupValue(identifier, false);
		if (bs != null)
			return copyBitSet(bs);

		// next we look for names of groups (PDB) or atoms (non-PDB)
		bs = viewer.getAtomBits("IdentifierOrNull", identifier);
		return (bs == null ? new BitSet() : bs);
	}