public void addArg(final BOp newArg){
  if (newArg == null)   throw new IllegalArgumentException();
  if (newArg == this)   throw new IllegalArgumentException();
  args.add(newArg);
  mutation();
}

public void addArg ( final BOp newArg ) { if ( newArg == null ) throw new IllegalArgumentException ( ) ; if ( newArg == this ) throw new IllegalArgumentException ( ) ; args . add ( newArg ) ; mutation ( ) ; }
add a new argument ( core mutation method ) .
