public double signed_area(PlaPointInt p_1,PlaPointInt p_2){
  PlaVectorInt d21=p_2.difference_by(p_1);
  PlaVectorInt d01=difference_by(p_1);
  return d21.determinant(d01);
}

public double signed_area ( PlaPointInt p_1 , PlaPointInt p_2 ) { PlaVectorInt d21 = p_2 . difference_by ( p_1 ) ; PlaVectorInt d01 = difference_by ( p_1 ) ; return d21 . determinant ( d01 ) ; }
returns the signed area of the parallelogramm spanned by the vectors p_2 - p_1 and this - p_1
