code: public TSIGRecord
  getTSIG() {
  	int count = header.getCount(Section.ADDITIONAL);
  	if (count == 0)
  		return null;
  	List l = sections[Section.ADDITIONAL];
  	Record rec = (Record) l.get(count - 1);
  	if (rec.type !=  Type.TSIG)
  		return null;
  	return (TSIGRecord) rec;
  }
docs: Returns the TSIG record from the ADDITIONAL section, if one is present.
  @see TSIGRecord
  @see TSIG
  @see Section