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import AvlVerification.Tree
namespace BST
open Primitives (Result)
open avl_verification (AVLNode Ordering)
open Tree (AVLTree AVLNode.left AVLNode.right AVLNode.val)
inductive ForallNode (p: T -> Prop): AVLTree T -> Prop
| none : ForallNode p none
| some (a: T) (left: AVLTree T) (right: AVLTree T) : ForallNode p left -> p a -> ForallNode p right -> ForallNode p (some (AVLNode.mk a left right))
-- This is the binary search invariant.
inductive BST [LT T]: AVLTree T -> Prop
| none : BST none
| some (a: T) (left: AVLTree T) (right: AVLTree T) :
ForallNode (fun v => v < a) left -> ForallNode (fun v => a < v) right
-> BST left -> BST right -> BST (some (AVLNode.mk a left right))
@[simp]
theorem singleton_bst [LT T] {a: T}: BST (some (AVLNode.mk a none none)) := by
apply BST.some
all_goals simp [ForallNode.none, BST.none]
theorem left [LT T] {t: AVLTree T}: BST t -> BST t.left := by
intro H
induction H with
| none => exact BST.none
| some _ _ _ _ _ _ _ _ _ => simp [AVLTree.left]; assumption
theorem right [LT T] {t: AVLTree T}: BST t -> BST t.right := by
intro H
induction H with
| none => exact BST.none
| some _ _ _ _ _ _ _ _ _ => simp [AVLTree.right]; assumption
end BST
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