(** 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).