code: func emailUsername(email string) string {
  	at := strings.Index(email, "@")
  	if at == -1 {
  		return email
  	} else if at == 0 {
  		return email[1:]
  	}
  	return email[:at]
  }
docs: // emailUsername returns the username portion of an email address (part before
  // '@') or the original input if it can't find the "@" symbol.