anchor.diff:    (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)) ")"))
  +                        (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))
  
positive.code:     (assert-type! xs list)
      (let* [(accum z)]
        (for i start (n xs) 1
          (set! accum (f accum (nth xs i))))
        accum)))
  
  (defun map (fn &xss)
  
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))
  