Opaque combine.

(* Given the relations on histories, what can we actually conclude about the maps? *)
Lemma make_map_eq : forall h h', Forall2 (fun a b => value_of_hist (fst a) = value_of_hist (fst b) /\
value_of_hist (snd a) = value_of_hist (snd b)) h h' -> make_map h = make_map h'.