Section LSort.
Variable A : Type.
Fixpoint alist_sorted (l:alist A) : bool :=
match l with
| nil => true
| (a, _) :: l' =>
match l' with
| nil => true
| (a', y') :: l'' => andb (Nless a a') (alist_sorted l')
end
end.
Fixpoint alist_nth_ad (n:nat) (l:alist A) {struct l} : ad :=
match l with
| nil => N0 (* dummy *)
| (a, y) :: l' => match n with
| O => a
| S n' => alist_nth_ad n' l'
end
end.
Definition alist_sorted_1 (l:alist A) :=
forall n:nat,
S (S n) <= length l ->
Nless (alist_nth_ad n l) (alist_nth_ad (S n) l) = true.
Definition alist_sorted_2 (l:alist A) :=
forall m n:nat,
m < n ->
S n <= length l -> Nless (alist_nth_ad m l) (alist_nth_ad n l) = true.

Lemma interval_split :
forall p q n:nat,
S n <= p + q -> {n' : nat | S n' <= q /\ n = p + n'} + {S n <= p}.