function two_point_line1(x0,y0,x1,y1) {
  return [x0,y0,x1 - x0,y1-y0];
}

function two_point_line2(x0,y0,x1,y1) {
  return {x:x0,y:y0,dx:x1-x0,dy:y1-y0}
}

function two_point_line3(x0,y0,x1,y1) {
  return {p:[x0,y0],d:[x1-x0,y1-y0]}
}