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: store.go
    path of file: ./repos/the-way-to-go_ZH_CN/eBook/examples/chapter_19/goto_v2
    
    the code of the file until where you have to start completion: package main

import (
	"encoding/gob"
	"io"
	"log"
	"os"
	"sync"
)

type URLStore struct {
	urls map[string]string
	mu   
    
func (s *URLStore) Set(key, url string) bool {
	s.mu.Lock()
	defer s.mu.Unlock()
	if _, present := s.urls[key]; present {
		return false
	}
	s.urls[key] = url
	return true
}