anchor.diff:     ```"
     (assert-type! f function)
     (assert-type! xs list)
  -  (cond
  -    [(! (exists? acc)) (map f xs '())]
  -    [(nil? xs) (reverse acc)]
  -    [true (map f (cdr xs) (cons (f (car xs)) acc))]))
  -
  -(defun filter (p xs acc)
  +  (let* [(out '())]
  +    (for i 1 (# xs) 1
  +      (push-cdr! out (f (nth xs i))))
  +    out))
  +
  +(defun filter (p xs)
     "Return the list of elements of XS which match the predicate P.
   
      Example:
  
positive.code:       (set! accum (f accum (nth xs i))))
      accum))
  
  (defun map (f xs acc)
    "Apply the function F to every element of the list XS, collecting the
     results in a new list.
  
  
negative.code:    > (zip list '(1 2 3) '(4 5 6) '(7 8 9))
     '((1 4 7) (2 5 8) (3 6 9))
     ```"
    (let* [(lenghts (map # xss))
           (out '())]
      (for i 1 (min (unpack lenghts)) 1
        (push-cdr! out (fn (unpack (nths xss i)))))
      out))
  
  ;; AUTOMATICALLY GENERATED
  ;; DO NOT EDIT please.
  