#lang pl
(: arith-op : (Number Number -> Number) FLANG FLANG -> FLANG)
;; gets a Racket numeric binary operator, and uses it within a FLANG
;; `Num' wrapper
(define (arith-op op expr1 expr2)
  (: Num->number : FLANG -> Number)
  (define (Num->number e)
    (cases e
      [(Num n) n]
      [else (error 'arith-op "expects a number, got: ~s" e)]))
  (Num (op (Num->number expr1) (Num->number expr2))))