proposition: Lemma nth_some : forall (l : list A) n a, nth_error l n = Some a -> n < length l.
proof: Proof. intros. apply (proj1 (nth_in l n)). rewrite H; discr. Qed.
imports: CoLoR(LogicUtil EqUtil RelUtil BoolUtil NatUtil) Coq(Setoid SetoidList FunInd)