(require "logic.BRU" "lists.BRU" "numbers.BRU" "pairs.BRU")
[(map-impl (\map (\callback (\list 
	(if (null? list)
	    nil
	    (cons (callback (head list)) (map callback (rest list))))))))

 (fold-impl (\fold (\base-case-value (\combiner (\ls
        (if (null? ls)
                base-case-value
                (combiner (head ls)
                        (fold base-case-value combiner (rest ls)))))))))

 ;(define sum (fold 0 +))
 ;(define product (fold 1 ∙))

 (length-impl (\length (\list
	(if (null? list)
		0
		(succ (length (rest list)))))))
 (zip-impl (\zip (\l1 (\l2 
       (if (null? l1)
           nil
           (cons (pair (head l1) (head l2)) (zip (rest l1) (rest l2))))))))]