anchor.diff:  (import core/base b)
   (import lua/table)
   (import lua/string)
  -(import core/type (nil? list? empty? assert-type! exists? falsey? eq? neq? type pretty))
  +(import core/type (nil? list? empty? assert-type! exists? falsey? eq? neq? type pretty debug))
   (import lua/math (min max huge))
   
   (defun car (x)
  
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:    out = (1 2 3 4)
     ```"
    (assert-type! xs list)
    (let* [(nxs (n xs))
           (len (+ nxs (n vals)))]
      (set-idx! xs "n" len)
      (for i 1 (n vals) 1
        (set-idx! xs (+ nxs i) (get-idx vals i)))
      xs))
  
  (defun pop-last! (xs)
  