intros context a b der_a.
elim der_a; clear der_a.
intros t der_t.
apply Derivable_Intro with (Shift t).
apply ShiftIntro; assumption.
Qed.

Lemma derivable_weak_list :
forall (context1 context2 : flist) (a : form),
Derivable context1 a -> Derivable (context2 ++ context1) a.