intros a gamma work c in_gamma0.
elim in_gamma0; clear in_gamma0 c.
intros n c nth.
apply In_Gamma with n; assumption.
intros n c nth.
apply In_Work1 with (S n).
apply My_NthS; assumption.
Qed.

(* Specifications: *)
Lemma in_gamma_cons_work_head :
forall (a : normal_form) (gamma : flist) (work : nf_list),
in_gamma gamma (a :: work) (nf2form a).