anchor.diff:      (assert-type! xs list)
       (let* [(accum z)]
         (for i start (n xs) 1
  -        (set! accum (f accum (nth xs i))))
  +           (set! accum (f accum (nth xs i))))
         accum)))
   
   (defun map (fn &xss)
  
positive.code:   (let* [(lengths (let* [(out '())]
                      (for i 1 (n xss) 1
                        (if (not (list? (nth xss i)))
                          (error (.. "not a list: " (pretty (nth xss i))
                                     " (it's a " (type (nth xss i)) ")"))
                          true)
                        (push-cdr! out (n (nth xss i))))
                      out))
  
negative.code:   (let* [(ns (let* [(out '())]
                      (for i 1 (n xss) 1
                        (if (not (list? (nth xss i)))
                          (error (.. "that's no list! " (pretty (nth xss i))
                                     " (it's a " (type (nth xss i)) "!)"))
                          true)
                        (push-cdr! out (n (nth xss i))))
                      out))
  