Definition hists_mono (h1 h2 : list (hist * hist)) := Forall2 (fun a b =>
(exists l, fst b = fst a ++ l) /\ (exists l, snd b = snd a ++ l)) h1 h2.
Instance hists_mono_refl : RelationClasses.Reflexive hists_mono.
Hint Resolve hists_mono_refl.
Instance hists_mono_trans : RelationClasses.Transitive hists_mono.

Lemma timestamp_unique : forall (h : list hist) l t e1 e2 i1 i2 (Hl : hist_list' (concat h) l)
(Ht1 : In (t, e1) (Znth i1 h [])) (Ht2 : In (t, e2) (Znth i2 h [])), i1 = i2.