Definition cdr (X : Set) (l : list X) :=
match l with
| Nil => Nil X
| Cons _ xs => xs
end.
Fixpoint length (X : Set) (l : list X) {struct l} : nat :=
match l with
| Nil => 0
| Cons _ xs => S (length X xs)
end.
(* append x y = yx *)
Fixpoint append (X : Set) (ys xs : list X) {struct xs} :
list X :=
match xs with
| Nil => ys
| Cons x xs' => Cons X x (append X ys xs')
end.
(* == Proprietes d'un tri == *)
Fixpoint sorted (l : list nat) : Prop :=
match l with
| Cons n (Cons m x as l) => inf_egal n m = true /\ sorted l
| _ => True
end.

Lemma sorted_eq3 :
forall (n m : nat) (x : list nat),
sorted (Cons nat n (Cons nat m x)) =
(inf_egal n m = true /\ sorted (Cons nat m x)).