proposition: Lemma tnth_behead n T (t : n.+1.-tuple T) i : tnth [tuple of behead t] i = tnth t (inord i.+1).
proof: Proof. by case/tupleP: t => x t; rewrite !(tnth_nth x) inordK ?ltnS. Qed.
imports: HB(structures) mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat) mathcomp(seq choice fintype path)