Definition genvs_domain_eq {F1 F2 V1 V2: Type} (ge1: Genv.t F1 V1) (ge2: Genv.t F2 V2) :=
(forall b, fst (genv2blocks ge1) b <-> fst (genv2blocks ge2) b) /\
(forall b, snd (genv2blocks ge1) b <-> snd (genv2blocks ge2) b).

Lemma unchanged_on_trans: forall m1 m2 m3 U
(U1: Mem.unchanged_on U m1 m2)
(U2: Mem.unchanged_on U m2 m3)
(Fwd12: mem_forward m1 m2),
Mem.unchanged_on U m1 m3.