Section ISortExample.
Variable A : Type.
Variable order : A -> A -> Prop.
Variable order_fun : A -> A -> bool.
Hypothesis order_fun_true : forall a b : A, order_fun a b = true -> order a b.
Hypothesis order_fun_false : forall a b : A, order_fun a b = false -> order b a.
(* Insert an element *)
Fixpoint insert (a : A) (l : list A) {struct l} : list A :=
match l with
| nil => a :: nil
| b :: l1 =>
match order_fun a b with
| true => a :: l
| false => b :: insert a l1
end
end.
Proof using order_fun_false order_fun_true.
intros l H'; elim H'; clear H' l; auto.
simpl in |- *; auto.
intros a b; simpl in |- *.
generalize (refl_equal (order_fun b a));
pattern (order_fun b a) at -1 in |- *; case (order_fun b a);
intros Eq0; auto.
intros a b l H'0 H'1 H'2 a0.
simpl in |- *.
generalize (refl_equal (order_fun a0 a));
pattern (order_fun a0 a) at -1 in |- *; case (order_fun a0 a);
intros Eq0; auto.
generalize (H'2 a0); simpl in |- *.
generalize (refl_equal (order_fun a0 b));
pattern (order_fun a0 b) at -1 in |- *; case (order_fun a0 b);
intros Eq1; auto.
Qed.

(* Inserting returns a permutation *)
Theorem insert_permutation :
forall (L : list A) (a : A), permutation (a :: L) (insert a L).