anchor.diff:                          (floor pt1x) (floor pt1y)
                           (floor pt2x) (floor pt2y)))))
   
  -(defun draw-pt3d (canvas mat pt xmin xmax ymin ymax width height &optional (tick-size 0.125))
  +(declaim (inline draw-quad))
  +(defun draw-quad (canvas mat p1 p2 p3 p4)
  +  (declare (type matrix mat)
  +           (type pt3d p1 p2 p3 p4))
  +  (draw-line3d canvas mat p1 p2)
  +  (draw-line3d canvas mat p2 p3)
  +  (draw-line3d canvas mat p3 p4)
  +  (draw-line3d canvas mat p4 p1))
  +
  +(defun draw-pt3d (canvas mat pt &optional (tick-size 0.125))
     "Draw a point as a mini-axis."
     (let ((tp (create-pt3d (pt3d-x pt) (+ (pt3d-y pt) tick-size) (pt3d-z pt)))
           (bp (create-pt3d (pt3d-x pt) (- (pt3d-y pt) tick-size) (pt3d-z pt)))
  -
           (lp (create-pt3d (- (pt3d-x pt) tick-size) (pt3d-y pt) (pt3d-z pt)))
           (rp (create-pt3d (+ (pt3d-x pt) tick-size) (pt3d-y pt) (pt3d-z pt)))
  -
           (np (create-pt3d (pt3d-x pt) (pt3d-y pt) (+ (pt3d-z pt) tick-size)))
           (fp (create-pt3d (pt3d-x pt) (pt3d-y pt) (- (pt3d-z pt) tick-size))))
  -    (draw-line3d canvas mat tp bp xmin xmax ymin ymax width height)
  -    (draw-line3d canvas mat lp rp xmin xmax ymin ymax width height)
  -    (draw-line3d canvas mat np fp xmin xmax ymin ymax width height)))
  +    (draw-line3d canvas mat tp bp)
  +    (draw-line3d canvas mat lp rp)
  +    (draw-line3d canvas mat np fp)))
   
   (defun random-pt3d (&optional (sz 8.0))
     (ltk3d:create-pt3d (- (random sz) (/ sz 2.0)) (- (random sz) (/ sz 2.0))
  
positive.code:   (pt1 (make-pt3d) :type pt3d)
    (pt2 (make-pt3d) :type pt3d))
  
  (defmacro scale-val (val minval maxval minnew maxnew)
    `(let ((new-diff (- ,maxnew ,minnew))
           (old-diff (- ,maxval ,minval))
           (val-diff (- ,val ,minval)))
       (+ (* (/ val-diff old-diff) new-diff) ,minnew)))
  
  (declaim (inline draw-line3d))
  (defun draw-line3d (canvas mat p1 p2 xmin xmax ymin ymax width height)
    (let ((tp1 (xform p1 mat))
          (tp2 (xform p2 mat)))
      (let ((pt1x (pt3d-x tp1))
  
negative.code:        (tol-equal (pt3d-y pt1) (pt3d-y pt2) eps)
         (tol-equal (pt3d-z pt1) (pt3d-z pt2) eps)
         (tol-equal (pt3d-w pt1) (pt3d-w pt2) eps)))
  
  (defun matrix= (m1 m2 &optional (eps 0.000001))
    (let ((good-so-far t))
  