proposition: Lemma all_tnthP a t : reflect (forall i, a (tnth t i)) (all a t).
proof: Proof. by rewrite -forallb_tnth; apply: forallP. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq choice fintype path)