anchor.diff:      (mapcar
        (lambda (polygon)
          (let ((sign
  -              (some (lambda (pt)
  -                      (let ((pt-sign (line-equation-res line pt)))
  -                        (unless (= pt-sign 0)
  -                          pt-sign)))
  -                    (point-list polygon))))
  +              (nth-value 1 (find-non-collinear-point line polygon))))
            (if (>= (* sign point-sign) 0)
                polygon
                (mirror-polygon polygon line))))
  
positive.code: (defpackage :src/simple-state
    (:use :common-lisp
          :2d-geometry
          :src/drawer)
    (:import-from :2d-geometry
                  :point-equal-p))
  
  
negative.code:                (assert1 (mapcar #'polygon->list res) expected))))
      (%test :a 0 :b 1 :c -1
             :coords '(0 0 0 2 2 2 2 0)
             :expected '((0 1 0 2 2 2 2 1)
                         (0 0 0 1 2 1 2 0)))
      (%test :a 0 :b 1 :c -2
             :coords '(0 0 0 2 2 2 2 0)
             :expected '((0 0 0 2 2 2 2 0)))
      (%test :a -1 :b 1 :c (- (/ 1 4))
             :coords '(0 0 0 1 1 1 1 0)
             :expected '((0 1/4 0 1 3/4 1)
                         (0 0 0 1/4 3/4 1 1 1 1 0)))))
  
  (defun mirror-polygon (polygon line)
    (make-polygon-from-point-list
  