anchor.diff:                        radius-1-dx radius-1-dy
                         radius-2-dx radius-2-dy
                         &key start-angle end-angle)
  -  (make-ellipical-thing 'standard-ellipse
  +  (make-elliptical-thing 'standard-ellipse
                           center-x center-y
                           radius-1-dx radius-1-dy
                           radius-2-dx radius-2-dy
  
positive.code:         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
  
negative.code:                              radius-1-dx radius-1-dy
                               radius-2-dx radius-2-dy
                               &key start-angle end-angle)
    (make-elliptical-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-elliptical-thing (class
                               center-x center-y
                               radius-1-dx radius-1-dy
                               radius-2-dx radius-2-dy
  