Definition eqo i j :=
   (if i =? j as b return ((b = true -> i = j) -> option (i=j))
    then fun Heq : true = true -> i = j =>
             Some (Heq (eq_refl true))
     else fun _ : false = true -> i = j => None) (eqb_correct i j).

Lemma gcd_spec a b : Zis_gcd (φ a) (φ b) (φ (gcd a b)).