intros a gamma work.
apply In_Work1 with 0.
apply My_NthO.
Qed.

(* Specifications: *)
Lemma in_gamma_cons_work_rev :
forall (a : normal_form) (gamma : flist) (work : nf_list) (c : form),
in_gamma gamma (a :: work) c -> in_gamma gamma work c + {c = nf2form a}.