intros p q n Hle; simpl in |- *.
rewrite le_minus_O.
rewrite minus_plus.
auto.
auto with arith.
Qed.

(* Specifications: *)
Theorem construct_correct :
forall (w : Qpositive) (p q n : nat),
Qpositive_i w = (p, q) -> p + q <= n -> Qpositive_c p q n = w.