intros context b c der.
elim der; clear der.
intros t der_t.
apply
Derivable_Intro with (App (Imp Falsum b) t (Abs Falsum (Efq (Var 0) b))).
apply ImpElim.
assumption.
apply ImpIntro.
apply ByAbsurdity.
apply ByAssumption; apply My_NthO.
Qed.

Lemma derivable_b__derivable_a_imp_b :
forall (a b : form) (context : flist),
Derivable context b -> Derivable context (Imp a b).