public PlaDirection add(PlaDirection p_other){
  long new_x=dir_x + p_other.dir_x;
  long new_y=dir_y + p_other.dir_y;
  return new PlaDirection(BigInteger.valueOf(new_x),BigInteger.valueOf(new_y));
}

public PlaDirection add ( PlaDirection p_other ) { long new_x = dir_x + p_other . dir_x ; long new_y = dir_y + p_other . dir_y ; return new PlaDirection ( BigInteger . valueOf ( new_x ) , BigInteger . valueOf ( new_y ) ) ; }
return a new pladirection that is the sum of this direction plus the other one
