code: @Override
  	public Group getGroupByPDB(ResidueNumber resNum) throws StructureException {
  		String pdbresnum = resNum.toString();
  		if ( pdbResnumMap.containsKey(pdbresnum)) {
  			Integer pos = pdbResnumMap.get(pdbresnum);
  			return groups.get(pos);
  		} else {
  			throw new StructureException("unknown PDB residue number " + pdbresnum + " in chain " + authId);
  		}
  	}
docs: {@inheritDoc}