(* This map instance requires that maps be disjoint, providing e.g. uniqueness of timestamps for histories. *)
Definition disjoint m1 m2 := forall k v1, m1 k = Some v1 -> m2 k = None.
Global Instance disjoint_comm : Symmetric disjoint.

Lemma disjoint_incl : forall m1 m2 m (Hcompat : disjoint m2 m) (Hincl : map_incl m1 m2), disjoint m1 m.