Definition jmeq2 A1 A2 B1 B2 (x : T A1 B1) (y : T A2 B2) :=
             forall pf, coerce2 x pf = y.

(* Specifications: *)
Lemma jm2E A B (x y : T A B) : jmeq2 x y <-> x = y.
Lemma refl_jmeq2 A B (x : T A B) : jmeq2 x x.