Section ListDenoteLems.
Context {A T : Type}.
Variables (f : A -> T) (g : T -> T -> T) (b : T).
Proof with auto.
induction l; simpl... rewrite IHl, listd_app; apply H.
Qed.

Lemma listd_AS_unit un l (un_unit : forall x, g x un = x) :
listd f g b l = g b (listd f g un l).