(* step 4 *)

(* Positivity predicate over points *)
Definition is_posp (c : R * R) := (fst c >= 0)%R /\ (snd c >= 0)%R.(* Predicate for the positive rational coordinates of the unit circle *)
Definition is_ucp (c : R * R) :=
  (in_ucirc (fst c) (snd c)) /\ (is_ratp c) /\ (is_posp c).(* Positive rational coordinates of the unit circle *)
Definition ucp (p q : Z) :=
  let pr := (IZR p) in
  let qr := (IZR q) in
  let den := (pr * pr + qr * qr)%R in
  ((qr * qr - pr * pr) / den, (2 * pr * qr) / den)%R.(* A basic condition over p and q *)
Definition cond_pqb (p q : Z) := p >= 0 /\ q > 0 /\ p <= q /\ (rel_prime p q).(* Set of positive rational coordinates of the unit circle *)
Definition in_ucp_setb (x y : R) :=
  exists p : Z, exists q : Z,
  x = (fst (ucp p q)) /\ y = (snd (ucp p q)) /\ (cond_pqb p q).

Lemma rat_pos_coo2 : forall x y : R, (is_ucp (x,y)) -> (in_ucp_setb x y).