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

Lemma T2 : forall (q r:Prop),(((r ≡≡ (q ≡≡ r)) ≡≡ ((r ≡≡ q) ≡≡ r)) ≡≡ ((q ≡≡ r) ≡≡ (r ≡≡ q))).