anchor.diff:     10
      ```"
     (get-idx xs idx))
  +
  +(defun nths (xss idx)
  +  "Get the IDX-th element in all the lists given at XSS. The first
  +   element is1.
  +
  +   Example:
  +   ```
  +   > (nths '((1 2 3) (4 5 6) (7 8 9)) 2)
  +   '(2 5 8)
  +   ```"
  +  (let* [(out '())]
  +    (for i 1 (# xss) 1
  +      (push-cdr! out (nth (nth xss i) idx)))
  +    out))
   
   (defun push-cdr! (xs val)
     "Mutate the list XS, adding VAL to its end.
  
positive.code:    ```"
    (assert-type! p function)
    (assert-type! xs list)
    (cond
      [(falsey? acc) (filter p xs '())]
      [(nil? xs) (reverse acc)]
      [true (if (p (car xs))
              (filter p (cdr xs) (cons (car xs) acc))
              (filter p (cdr xs) acc))]))
  
  (defun any (p xs)
    "Check for the existence of an element in XS that matches the
  
negative.code: (import base)
  (import lua/table)
  (import type (list? nil? assert-type! exists? falsey? eq?))
  (import lua/math (min max))
  
  (defun car (x)
    "Return the first element present in the list X. This function operates
  