code: func Subdomain(url string) string {
  	domain, top := stripURLParts(url), Domain(url)
  	lt, ld := len(top), len(domain)
  	if lt < ld && top != "" {
  		return domain[:(ld-lt)-1]
  	}
  	return ""
  }
docs: // Subdomain returns subdomain from provided url.
  // If subdomain is not found in provided url, this function returns empty string.