;; '(case s [x e] ...)
;; ==>
;; '(let ([tmp s]) (cond [(symbol=? tmp 'x) e] ...))
(define (case-transformer s)
  (match s
    [`(case ,subj [,sym* ,bdy*] ...)
     (let ([tmp (fresh! 'tmp)])
       `(let ([,tmp ,subj])
          (cond
            ,@(map (λ (sym bdy)
                     `[(symbol=? ,tmp ',sym) ,bdy])
                   sym*
                   bdy*))))]

    [_ (error 'case-transformer "malformed case: ~e" s)]))

#;; in regular racket:
(define-syntax (case stx)
  (syntax-case stx ()
    [(case subj [sym bdy] ...)
     ;; note: the syntax/parse library has a more convenient
     ;; form for with-syntax
     (with-syntax ([tmp (generate-temporary #'subj)])
       ;; "pattern variables" (subj,sym,bdy,tmp) are automatically
       ;; substituted and cant be used outside of syntax quote #'
       #'(let ([tmp subj])
           (cond
             ;; these ellipsis take care of mapping
             [(symbol=? tmp 'sym) bdy] ...)))]))