anchor.diff:  
   (defun brute-force-solve (cards &optional pre)
     (iter (for i from 0 to (length cards))
  -        $next-pre (append (copy-list pre) (list i))
  +        $(next-pre (append (copy-list pre) (list i)))
           (handler-case
               (try-removes next-pre cards)
             (simple-error () (next-iteration)))
           (if (= (length cards) (length next-pre))
               (return-from brute-force-solve next-pre)
  -            $recret (brute-force-solve cards next-pre)
  +            $(recret (brute-force-solve cards next-pre))
               (when (not (null recret))
                 (return-from brute-force-solve recret))))
     nil)
  
positive.code:   (island-fails))
  
  (defun try-removes (removes cards)
    $my-cards (copy-list cards)
    (dolist (remove removes)
      (remove-card remove my-cards)
      (when (unsolvable-p my-cards)
  
negative.code:   cards)
  
  (defun unsolvable-p (list)
    $(island-1 nil)
    $(in-island nil)
    $#(island-fails ()
       (if in-island
           (if island-1
               (clearf island-1 in-island)
               t)
           nil))
    (dolist (x list)
          (if (null x)
              (when (island-fails)
  