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

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