proposition: Lemma cycle_from_prev : (forall x, x \in p -> e (prev p x) x) -> cycle e p.
proof: Proof. move=> e_p; apply: cycle_from_next => x. by rewrite -mem_next => /e_p; rewrite prev_next. Qed.
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat seq)