Fixpoint Concat (l : Tlist_setoid) : Tlist_setoid -> Tlist_setoid :=
fun m : Tlist_setoid =>
match l with
| Empty => m
| Concat1 a l1 => Concat1 a (Concat l1 m)
end.

Lemma Diff_Concat1_Empty :
forall (a : A) (l : Tlist), ~ Equal_Tlist (Concat1 a l) Empty.