;;; Depende se o ciclo e' orientado ou nao
;;; Em ambos os casos, creio que nao se deve
;;; repetir arestas nem vertices intermediarios

(defun cycle (g a)
  "It depends on whether we are given a directed or undirected graph.
Moreover, I think that directed cycles must have at least one edge, and
undirected ones must have at least three edges.  In both cases, I think
one should not repeat intermediate vertices."
  )