Definition psub (p: aptr) (n: ptrofs) : aptr :=
  match p with
  | Gl id ofs => Gl id (Ptrofs.sub ofs n)
  | Stk ofs => Stk (Ptrofs.sub ofs n)
  | _ => p
  end.

Lemma psub_sound: forall b ofs p delta, pmatch b ofs p -> pmatch b (Ptrofs.sub ofs delta) (psub p delta).