Inductive set_op := Add (e : Z) | Remove (e : Z).Inductive set_op := Add (e : Z) | Remove (e : Z).Inductive set_op := Add (e : Z) | Remove (e : Z).

Lemma apply_hist_app : forall h1 S h2, apply_hist S (h1 ++ h2) = apply_hist (apply_hist S h1) h2.