下面程序是一个二叉排序树的插入实现，横线处应该填入的是：

class BinarySortTree:
    def __init__(self):
        self.root = None

    def insert(self, key, value):
        node = TreeNode(key, value)
        if self.root is None:
            self.root = node
            return
        current = self.root
        while True:
            if key < current.key:
                ___________________________
                current = current.left
            else:
                if current.right is None:
                    current.right = node
                    return
                current = current.right

    def search(self, key):
        current = self.root
        while current:
            if current.key == key:
                return current.value
            elif current.key > key:
                current = current.left
            else:
                current = current.right
        return None

    def inorder_traversal(self, node):
        if node:
            self.inorder_traversal(node.left)
            print(node.key, node.value)
            self.inorder_traversal(node.right)