Qed.
Fixpoint lt_bool (n m: nat) {struct n}: bool :=
match n, m with
O, (S _)  => true
| S n1, S m1 => lt_bool n1 m1
| _, _ => false
end.

Theorem lt_correct: forall n m, lt_bool n m = true -> n <= m.
induction n; destruct m; simpl; auto with arith; intros; discriminate.