(define-syntax assert
  (syntax-rules ()
    ((_ check body)
     (let ((result body))
       (when (not (check result))
         (error "assertion error: " check body result))
       result))))

(define-syntax asserts
  (syntax-rules ()
    ((_ (check ...) body)
     (let ((result body))
       (when (not (check result)) (error "assertion error: " check body result)) ...
       result))))