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

Lemma derivable_a_and_b__derivable_b :
forall (a b : form) (context : flist),
Derivable context (AndF a b) -> Derivable context b.