;;;;;
;;;;;
;;;;; Math
;;;;;
;;;;;

(define $fact
  (lambda [$n]
    (foldl * 1 (between 1 n))))

(define $perm
  (lambda [$n $r]
    (foldl * 1 (between (- n (- r 1)) n))))

(define $comb
  (lambda [$n $r]
    (/ (perm n r)
       (fact r))))