intros a context der.
elim der; clear der.
intros t der_t.
apply Derivable_Intro with (Cas Falsum a t (Efq (Var 0) a) (Var 0)).
apply OrFElim.
assumption.
apply ByAbsurdity.
apply ByAssumption.
apply My_NthO.
apply ByAssumption.
apply My_NthO.
Qed.

(* Specifications: *)
Lemma derivable_a_or_falsum__derivable_a :
forall (a : form) (context : flist),
Derivable context (OrF a Falsum) -> Derivable context a.