apply Derivable_Intro with (Abs a (Var 0)).
apply ImpIntro.
apply ByAssumption.
apply My_NthO.
Qed.

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