public void addArgIfAbsent(final BOp arg){
  if (arg == null)   throw new IllegalArgumentException();
  if (arg == this)   throw new IllegalArgumentException();
  if (!args.contains(arg)) {
    addArg(arg);
  }
}

public void addArgIfAbsent ( final BOp arg ) { if ( arg == null ) throw new IllegalArgumentException ( ) ; if ( arg == this ) throw new IllegalArgumentException ( ) ; if ( ! args . contains ( arg ) ) { addArg ( arg ) ; } }
add an argument iff it is not already present .
