(setf (vertex-visited v) t)
(setf (vertex-dfn v) (incf dfn))
(dolist (e (vertex-edges v))
(unless (vertex-visited (edge-vertex e))
(setf (vertex-visited v) t)
(setf (vertex-dfn v) (incf dfn))
(dolist (e (vertex-edges v))
(unless (vertex-visited (edge-vertex e))