anchor.diff:          radius-2-dy (coordinate radius-2-dy)
           start-angle (and start-angle (coordinate start-angle))
           end-angle (and end-angle (coordinate end-angle)))
  -
     (let ((tr (make-3-point-transformation*
                0 0 1 0 0 1
                center-x center-y
  
positive.code:                              radius-1-dx radius-1-dy
                               radius-2-dx radius-2-dy
                               &key start-angle end-angle)
    (make-ellipical-thing 'standard-elliptical-arc
                          center-x center-y
                          radius-1-dx radius-1-dy
                          radius-2-dx radius-2-dy
                          start-angle end-angle))
  
  (defun make-ellipical-thing (class
                               center-x center-y
                               radius-1-dx radius-1-dy
                               radius-2-dx radius-2-dy
  
negative.code:                       radius-1-dx radius-1-dy
                        radius-2-dx radius-2-dy
                        &key start-angle end-angle)
    (make-elliptical-thing 'standard-ellipse
                          center-x center-y
                          radius-1-dx radius-1-dy
                          radius-2-dx radius-2-dy
  