anchor.diff:    (for i (# li) 1 -1
       (set! accum (fn (get-idx li i) accum)))
     accum)
  +
  +(defun latter (x xs)
  +  (cond
  +    [(null? xs) xs]
  +    [true (let [(head (car xs))
  +                (tail (cdr xs))]
  +            (if (eq? head x)
  +              tail
  +              (latter x tail)))]))
  
positive.code:               gensym get-idx set-idx! list
                = > + -))
  (import base)
  (import types (assert-type!))
  
  
  (defun nth (li idx)
    (assert-type! li "list")
  
negative.code: 
  ;; Checks this is a list and is empty
  (defun nil? (li) (= (# li) 0))
  
  ;; Removes nils (i.e. '(), not nil) from a list
  (defun prune (li)
    (filter (lambda (x) (! (null? x))) li))
  
  ;; Perform an action for every entry in the list
  (defmacro for-each (var lst &body)
  