(* elimination rules for equivalence *)
Parameter detachR     : forall (p q:Prop), (p ≡≡ q) -> q -> p.
(* substitution rules for equivalence -> use rewrite *)

Lemma T4 : forall p q:Prop, ((p ≡≡ q) ≡≡ (q ≡≡ p)). (* symmetry in Proto *)