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

Lemma in_gamma_shift_gamma_work :
forall (a : normal_form) (gamma : flist) (work : nf_list) (c : form),
in_gamma (nf2form a :: gamma) work c -> in_gamma gamma (a :: work) c.