Golang Tour Webcrawler exercise

I am new in Go and for study I have to hold a presentation about concurrency in Go. I think the Golang Tour - Webcrawler exercise is a nice example to talk about that. Before I do that, it would be nice if anybody could verify if this solution fits. I assume it is correct but perhaps I have missed anything or any of you have a better alternative.

```
package main

import (
"fmt"
"sync"
"strconv"
"time"
)

/*
* Data and Types
* ===================================================================================
*/
var fetched map[string]bool // Map of fetched URLs -> true: fetched
var lock sync.Mutex // locks write access to fetched-map
var urlChan chan string // Channel to Write fetched URL

type Fetcher interface {
// Fetch returns the body of URL and
// a slice of URLs found on that page.
Fetch(url string) (body string, urls []string, err error)
}

// fakeFetcher is Fetcher that returns canned results.
type fakeFetcher map[string]*fakeResult

type fakeResult struct {
body string
urls []string
}

// fetcher is a populated fakeFetcher.
var fetcher = fakeFetcher{
"http://golang.org/": &fakeResult{
"The Go Programming Language",
[]string{
"http://golang.org/pkg/",
"http://golang.org/cmd/",
},
},
"http://golang.org/pkg/": &fakeResult{
"Packages",
[]string{
"http://golang.org/",
"http://golang.org/cmd/",
"http://golang.org/pkg/fmt/",
"http://golang.org/pkg/os/",
},
},
"http://golang.org/pkg/fmt/": &fakeResult{
"Package fmt",
[]string{
"http://golang.org/",
"http://golang.org/pkg/",
},
},
"http://golang.org/pkg/os/": &fakeResult{
"Package os",
[]string{
"http://golang.org/",
"http://golang.org/pkg/",
},
},
}
/*
* End Data and Types
* =================================

Combine the mutex with the thing it is locking into a single struct. Often mutex elements of a struct are unnamed, since you only need to lock and unlock the whole thing

type SafeMap struct {
  sync.Mutex
  URLs map[string]bool
}

which you can then use like so:

url := "http://some_example.com"
fetched := SafeMap{URLs:map[string]bool{}}
fetched.Lock()
fetched.URLs[url] = true
fetched.Unlock()

StackExchange Code Review Q#125087, answer score: 7