intros gamma work context k k_is_mon k_forces_gamma minimal0 a der_a.
elim der_a; clear der_a.
intros t der_t.
apply soundness_t with t context; try assumption.
intros c in_c.
apply minimal0; assumption.
Qed.
(*************************************************************************)

Lemma minimal_shift_gamma_work :
forall (a : normal_form) (gamma : flist) (work : nf_list) (context : flist),
minimal (nf2form a :: gamma) work context ->
minimal gamma (a :: work) context.