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 add_item_trace_mono : forall h k v i s h', add_item_trace h k v i s h' -> hists_mono h h'.