Corollary add_events_dom : forall h le h' t e, add_events h le h' -> h' t = Some e ->
  h t = Some e \/ In e le.Corollary add_events_incl : forall h le h', add_events h le h' -> map_incl h h'.Corollary add_events_newer : forall h le h' t, add_events h le h' -> newer h' t -> newer h t.

(* Specifications: *)
Lemma add_events_in : forall h le h' e, add_events h le h' -> In e le -> exists t, newer h t /\ h' t = Some e.