proposition: Lemma has_tnthP a t : reflect (exists i, a (tnth t i)) (has a t).
proof: Proof. by rewrite -existsb_tnth; apply: existsP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq choice fintype path)