(** Least upper bound *)

Definition combine_ablock (ob1 ob2: option ablock) : option ablock :=
  match ob1, ob2 with
  | Some b1, Some b2 => Some (blub b1 b2)
  | _, _ => None
  end.Definition mlub (m1 m2: amem) : amem :=
{| am_stack := blub m1.(am_stack) m2.(am_stack);
   am_glob  := PTree.combine combine_ablock m1.(am_glob) m2.(am_glob);
   am_nonstack := plub m1.(am_nonstack) m2.(am_nonstack);
   am_top := plub m1.(am_top) m2.(am_top) |}.

Lemma mmatch_lub_l: forall m x y, mmatch m x -> mmatch m (mlub x y).