Section Tree.
Variable A : Type.
Variable eqA_dec : forall a b : A, {a = b} + {a <> b}.
Variable empty : A.
(* Definition of binary trees *)
Inductive btree : Type :=
| leaf : A -> btree
| node : btree -> btree -> btree.
(* Predicate that defines belonging *)
Inductive inb : btree -> btree -> Prop :=
| inleaf : forall t : btree, inb t t
| innodeL : forall t t1 t2 : btree, inb t t1 -> inb t (node t1 t2)
| innodeR : forall t t1 t2 : btree, inb t t2 -> inb t (node t1 t2).
Hint Constructors inb : core.
Proof using.
intros t1 t2 t3 H H1; generalize t1 H; elim H1; clear H H1 t1 t2 t3; auto.
Qed.
Proof using.
intros t; elim t; simpl in |- *; auto.
intros a; exists a; auto.
intros b (a, H) b0 H0; exists a; auto.
Qed.
(* Compute the number of nodes of a tree *)
Fixpoint number_of_nodes (b : btree) : nat :=
match b with
| leaf _ => 0
| node b1 b2 => S (number_of_nodes b1 + number_of_nodes b2)
end.
Proof using.
intros t1 t2 H; elim H; clear H t1 t2; simpl in |- *; auto.
intros t t1 t2 H H0; apply le_trans with (1 := H0); auto with arith.
intros t t1 t2 H H0; apply le_trans with (1 := H0); auto with arith.
Qed.
Proof using.
intros t1 t2 H; elim H; auto.
intros t t0 t3 H0 H1 H2.
absurd (number_of_nodes (node t0 t3) <= number_of_nodes t).
rewrite H1; simpl in |- *; auto with arith.
apply inb_trans with (2 := H2); auto.
apply number_of_nodes_inb_le; auto.
intros t t0 t3 H0 H1 H2.
absurd (number_of_nodes (node t0 t3) <= number_of_nodes t).
rewrite H1; simpl in |- *; auto with arith.
apply inb_trans with (2 := H2); auto.
apply number_of_nodes_inb_le; auto.
Qed.
(* equality on trees is decidable  *)
Definition btree_dec : forall a b : btree, {a = b} + {a <> b}.
intros a; elim a.
intros a1 b; case b.
intros b1; case (eqA_dec a1 b1).
intros e; left; rewrite e; auto.
intros e; right; Contradict e; inversion e; auto.
intros b0 b1; right; red in |- *; intros H; discriminate.
intros b H b0 H0 b1; case b1.
intros a0; right; red in |- *; intros H1; discriminate.
intros b2 b3; case (H b2); intros H1.
case (H0 b3); intros H2.
left; rewrite H1; rewrite H2; auto.
right; rewrite H1; Contradict H2; inversion H2; auto.
right; Contradict H1; inversion H1; auto.
Defined.
(* Belonging is decidable *)
Definition inb_dec : forall a p, {inb a p} + {~ inb a p}.
intros a; elim a; simpl in |- *; auto; clear a.
intros a p; elim p; simpl in |- *; auto; clear p.
intros a1; case (eqA_dec a a1); intros Ha.
left; rewrite Ha; simpl in |- *; auto.
right; red in |- *; Contradict Ha; inversion Ha; auto.
intros b [H| H]; auto.
intros b0 [H1| H1]; auto.
right; red in |- *; intros H2; inversion H2.
case H; auto.
case H1; auto.
intros b H b0 H0 p; elim p; auto.
intros a; right; red in |- *; intros H1; inversion H1.
intros b1 H1 b2 H2.
case H1; intros H3; auto.
case H2; intros H4; auto.
case (btree_dec (node b b0) (node b1 b2)); intros H5.
left; rewrite H5; auto.
right; red in |- *; intros H6; inversion H6; auto.
case H5; rewrite H9; rewrite H10; auto.
Defined.
(* Compute all the leaves of the tree *)
Fixpoint all_leaves (t : btree) : list A :=
match t with
| leaf a => a :: nil
| node t1 t2 => all_leaves t1 ++ all_leaves t2
end.
Proof using.
intros t; elim t; simpl in |- *; auto.
intros a a0 H; inversion H; auto.
intros b H b0 H0 a H1; apply in_or_app; inversion H1; auto.
Qed.
Proof using.
intros t; elim t; simpl in |- *; auto.
intros a a0 [H| H]; [ rewrite H | case H ]; auto.
intros b H b0 H0 a H1; case in_app_or with (1 := H1); auto.
Qed.
(* The property for a tree to have distinct leaves  *)
Definition distinct_leaves (t : btree) : Prop :=
forall t0 t1 t2 : btree,
inb (node t1 t2) t -> inb t0 t1 -> inb t0 t2 -> False.
Proof using.
intros a; red in |- *.
intros a0 t1 t2 H; inversion H.
Qed.
Hint Resolve distinct_leaves_leaf : core.
Proof using.
intros t1 t2 H; red in |- *.
intros a t0 t3 H0 H1 H2.
apply (H a t0 t3); auto.
Qed.

(* An inversion theorem for node *)
Theorem distinct_leaves_r :
forall t1 t2 : btree, distinct_leaves (node t1 t2) -> distinct_leaves t2.