From Undecidability.L.Tactics Require Import LTactics.
From Undecidability.L Require Import UpToC.
From Undecidability.L.Datatypes Require Export List_enc List_in List_basics LBool LNat.
Set Default Proof Using "Type".
Definition lengthEq A :=
fix f (t:list A) n :=
match n,t with
0,nil => true
| (S n), _::t => f t n
| _,_ => false
end.

Lemma lengthEq_spec A (t:list A) n:
| t | =? n = lengthEq t n.