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_union: forall m m' P Q (HP: Mem.unchanged_on P m m') (HQ: Mem.unchanged_on Q m m') (PQ: block -> Z -> Prop) (HPQ: forall b ofs, PQ b ofs -> P b ofs \/ Q b ofs), Mem.unchanged_on PQ m m'.