intros a gamma work c in_gamma0.
elim in_gamma0; clear in_gamma0 c.
intros n c; case n; clear n.
intros nth.
right; inversion_clear nth; trivial.
intros n nth.
left; apply In_Gamma with n.
inversion_clear nth; assumption.
intros n c nth.
left; apply In_Work1 with n; assumption.
Qed.

(* Specifications: *)
Lemma in_gamma_cons_work_tail :
forall (a : normal_form) (gamma : flist) (work : nf_list) (c : form),
in_gamma gamma work c -> in_gamma gamma (a :: work) c.