(define-syntax topo
  (syntax-rules ()
    ((topo ((n (m ...)) ...))
       (letrec ((n (lambda ()
                     (set! n (lambda () (error 'n "is part of a cycle")))
                     (m) ...
                     (set! n (lambda () "done")) 
	             (display 'n)(display " ")))
                ...)
              (n) ... ))))

(define test
   (lambda ()
     (topo ((a (b c e)) (b (d f)) (c (b)) (d ()) (e (b f)) (f (d))))))