You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: bst.go
    path of file: ./repos/codeforces-go/copypasta
    
    the code of the file until where you have to start completion: package copypasta

import (
	. "fmt"
	"strings"
)

// 二叉树常用函数

type bstNode struct {
	lr    [2]*bstNode
	sz    int
	msz   int
	key   int
	value int

    
func (t *bst) lowerBound(key int) (lb *bstNode) {
	for o := t.root; o != nil; {
		switch c := o.cmp(key); {
		case c == 0:
			lb = o
			o = o.lr[0]
		case c > 0:
			o = o.lr[1]
		default:
			return o
		}
	}
	return
}