(** ** Generating data types are in bijection to nat *)

(* Specifications: *)
Lemma lt_acc n :
Acc lt n.
Lemma LL_f n :
f n el LL (S n).
Lemma LL_F x n :
x el LL n -> exists m, F m = x.
Lemma F_sur :
forall x, exists n, F n = x.