func pageIDs(body string) (ids []string) {
	mv := idRx.FindAllStringSubmatch(body, -1)
	for _, m := range mv {
		ids = append(ids, m[1])
	}
	return
}

// url may contain a #fragment, and the fragment is then noted as needing to exist.
func crawl(url string, sourceURL string) {
	if strings.Contains(url, "/devel/release") {
		return
	}
	mu.Lock()
	defer mu.Unlock()