(** The default abstraction: if the result is unused, the arguments are
  unused; otherwise, the arguments are needed in full. *)

Definition default (x: nval) :=
  match x with
  | Nothing => Nothing
  | _ => All
  end.

Lemma default_needs_of_operation_sound: forall op args1 v1 args2 nv, eval_operation ge (Vptr sp Ptrofs.zero) op args1 m1 = Some v1 -> vagree_list args1 args2 nil \/ vagree_list args1 args2 (default nv :: nil) \/ vagree_list args1 args2 (default nv :: default nv :: nil) -> nv <> Nothing -> exists v2, eval_operation ge (Vptr sp Ptrofs.zero) op args2 m2 = Some v2 /\ vagree v1 v2 nv.