intros context a0 a1 b der.
elim der; clear der.
intros t der_t.
apply
Derivable_Intro with (Abs a1 (App (OrF a0 a1) (Shift t) (OrFR (Var 0) a0))).
apply ImpIntro.
apply ImpElim.
apply ShiftIntro; assumption.
apply OrFIntroR.
apply ByAssumption; apply My_NthO.
Qed.

Lemma derivable_falsum_imp_b_imp_c__derivable_c :
forall (context : flist) (b c : form),
Derivable context (Imp (Imp Falsum b) c) -> Derivable context c.