elim calc_dec with c c0; intros.
left; elim a; auto with arith pts.
right; red in |- *; intros H; apply b; injection H; auto with arith pts.
elim eq_nat_dec with n n0; intros.
left; elim a; auto with arith pts.
right; red in |- *; intros H; apply b; injection H; auto with arith pts.
Qed.

Lemma univ_v6_dec : forall s s' : srt_v6, decide (univ s s').
refine
(fun s s' : srt_v6 =>
match s, s' return (decide (univ s s')) with
| Sprop c, Sprop c' => _
| Sprop _, Stype _ => left _ _
| Stype n, Stype n' => _
| Stype n, Sprop c => right _ _
end).