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.