code: public static final Group getGroupByPDBResidueNumber(Structure struc,
  			ResidueNumber pdbResNum) throws StructureException {
  		if (struc == null || pdbResNum == null) {
  			throw new IllegalArgumentException("Null argument(s).");
  		}
  
  		Chain chain = struc.getPolyChainByPDB(pdbResNum.getChainName());
  
  		return chain.getGroupByPDB(pdbResNum);
  	}
docs: Get a group represented by a ResidueNumber.
  
  @param struc
  a {@link Structure}
  @param pdbResNum
  a {@link ResidueNumber}
  @return a group in the structure that is represented by the pdbResNum.
  @throws StructureException
  if the group cannot be found.