anchor.diff:     out = (1 2 3 4)
      ```"
     (assert-type! xs list)
  -  (let* [(len (+ (n xs) 1))]
  +  (let* [(nxs (n xs))
  +         (len (+ nxs (n vals)))]
       (set-idx! xs "n" len)
  -    (set-idx! xs len val)
  +    (for i 1 (n vals) 1
  +      (set-idx! xs (+ nxs i) (get-idx vals i)))
       xs))
   
   (defun pop-last! (xs)
  
positive.code:       (push-cdr! out (nth (nth xss i) idx)))
      out))
  
  (defun push-cdr! (xs val)
    "Mutate the list XS, adding VAL to its end.
  
     ### Example:
     ```cl
  
negative.code: (import core/base b)
  (import lua/table)
  (import lua/string)
  (import core/type (nil? list? empty? assert-type! exists? falsey? eq? neq? type pretty debug))
  (import lua/math (min max huge))
  
  (defun car (x)
  