Definition bij_sa_sum_option : bijection (A+unit) (option A) :=
    Bijection _ _ sa_sum_option option_sa_sum sa_sum__option option__sa_sum.

(* Specifications: *)
Lemma join_hom_list_nil : join_hom (fun a => a :: nil).
Lemma join_hom2_list_cons : join_hom2 (fun a l => a :: l).