Definition log_latest s (v1 : Z) (v2 : B) := s v1 = Some v2 /\ forall v', v1 < v' -> s v' = None.

Lemma log_latest_add : forall m1 m2 k1 k2 (v1 v2 : B) (Hlatest1 : log_latest m1 k1 v1) (Hlatest2 : log_latest m2 k2 v2), log_latest (map_add m1 m2) (Z.max k1 k2) (if zlt k1 k2 then v2 else v1).