Fixpoint Qpositive_c (p q n : nat) {struct n} : Qpositive :=
  match n with
  | O => One
  | S n' =>
      match p - q with
      | O => match q - p with
             | O => One
             | v => dL (Qpositive_c p v n')
             end
      | v => nR (Qpositive_c v q n')
      end
  end.

Theorem construct_correct4 : forall p q p' q' n n' : nat, S p + S q <= S n -> S p' + S q' <= S n' -> S p * S q' = S p' * S q -> Qpositive_c (S p) (S q) (S n) = Qpositive_c (S p') (S q') (S n').