anchor.diff:  (defun draw-field (scene)
     (let ((a1 *border*)
           (a2 (+ *border* *scale*))
  -        (a3 (round (/ *border* 2)))
  +        (a3 3)
           (step (round (/ *scale* *steps*)))
           (group
            (make-group scene (:stroke "black"
  
positive.code:   (let* ((polygons (src/types:silhouette problem))
           (lines (src/types:skeleton problem))
           (bounding-box (reduce #'bounding-box-union (mapcar #'cl-geometry::construct-bounding-box polygons)))
           (*x-shift* (/ (1+ (+ (cl-geometry::x-min bounding-box) (cl-geometry::x-max bounding-box))) 2))
           (*y-shift* (/ (1+ (+ (cl-geometry::y-min bounding-box) (cl-geometry::y-max bounding-box))) 2))
           (max-size (max (- (cl-geometry::x-max bounding-box) (cl-geometry::x-min bounding-box))
                          (- (cl-geometry::y-max bounding-box) (cl-geometry::y-min bounding-box))))
           (*border* (if (> max-size 1)
                         (round (/ (* *scale* (- max-size 1)) 2))
                         *border*))
           (size (+ *scale* (* 2 *border*))))
      (with-svg-to-file
  
negative.code:               (let ((problem (src/parser:parse-problem problem-file))
                      (svg-filename (format nil "~A.svg" problem-file)))
                  (draw-problem problem :filename svg-filename))
              (error (e) (format t "~A:~A~%" problem-file e))))
          (directory problem-folder)))
  
  