From Undecidability.L Require Import Util.L_facts.
Notation "A '.[' i  ']'" := (elAt A i) (no associativity, at level 50).
Fixpoint appCross A B :=
match A with
| nil => nil
| a :: A => map (app a) B ++ appCross A B
end.
Fixpoint T n :=
match n with
| 0 => [#n]
| S n => let A := T n in A ++ [#(S n)] ++ filter (fun x => Dec (~ x el A)) ( map lam A ++ appCross A A )
end.
Fixpoint exh_size s := match s with
| #n => n
| app s1 s2 => 1 + exh_size s1 + exh_size s2
| lam s => 1 + exh_size s
end.
Proof with
simpl; repeat rewrite in_app_iff; repeat rewrite in_filter_iff; try rewrite <- appCross_correct; eauto using in_filter_iff, in_app_iff, appCross_correct, el_elAt, elAt_el, T_ge, in_map.
induction s.
- destruct n; simpl; auto.
- eapply T_app; eapply T_el_ge; try eassumption; fold exh_size plus; lia.
- now eapply T_lam.
Qed.
induction m.
- simpl; lia.
- simpl. rewrite app_length. simpl. lia.
Qed.
Definition g s := match pos s (T (exh_size s)) with None => 0 | Some n => n end.
Definition g_inv n := match elAt (T n) n with None => #0 | Some n => n end.

Lemma g_inv_g s : g_inv (g s) = s.