(define (acyclic? graph)
  (if (null? graph)
      #t
      (let ((sink (find-sink graph)))
        (if sink 
            (acyclic? (remove-node (first sink) graph))
            #f))))

(define (find-sink graph)
  (if (null? graph)
      #f
      (if (sink? (first graph))
          (first graph)
          (find-sink (rest graph)))))

(define (sink? node)
  (null? (rest node)))

(define (remove-node sink graph)
  (if (null? graph)
      ()
      (if (equal? sink (first (first graph)))
          (remove-node sink (rest graph))
          (cons
           (remove-target sink (first graph))
           (remove-node sink (rest graph))))))

(define (remove-target sink node)
  (cons (first node) (remove-elements sink (rest node))))

(define (remove-elements sink L)
  (if (null? L)
      ()
      (if (equal? sink (first L))
          (remove-elements sink (rest L))
          (cons (first L) (remove-elements sink (rest L))))))

(define g1 '((a b c) (b c d) (c e) (d b e) (e)))
(define g2 '((a b c) (b c d) (c e) (d e) (e)))