Fixpoint app (a b: L) {struct a}: L :=
match a with
| one x => cons x b
| cons x y => cons x (app y b)
end.
Variable l: L.
Definition head := match l with one x => x | cons x _ => x end.
Definition tail := match l with one _ => List.nil | cons _ x => to_plain x end.

Lemma In_sound x: List.In x l -> (head = x \/ List.In x tail).
Proof with auto. unfold head. unfold tail. destruct l... Qed.