intros a b context der.
elim der; clear der.
intros t der_t.
apply Derivable_Intro with (PrR a t).
apply AndFElimR; assumption.
Qed.

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