Notation val_inject:=Val.inject.(*A value that is (if its a pointer) not dangling wrt m - a condition
 like this will probably be need to imposed on after-external return
 values (and thus also on the values returned by halted)*)
Definition val_valid (v:val) (m:mem):Prop :=
     match v with Vptr b ofs => Mem.valid_block m b | _ => True
     end.

Lemma extends_valvalid: forall m1 m2 (Ext: Mem.extends m1 m2) v, val_valid v m1 <-> val_valid v m2.