intros A B f o l0 l1 x f_perm.
elim l0; clear l0.
trivial.
intros a0 l0 ih.
simpl in |- *.
rewrite ih.
simpl in |- *.
apply f_perm.
Qed.
(********************************************************************)

Lemma plus_O : forall n : nat, n + 0 = n.
intros n.