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

Lemma T3 : forall (q r:Prop), ((r ≡≡ (q ≡≡ r)) ≡≡ ((r ≡≡ q) ≡≡ r)).