code: func (cl *Client) Challenge(serverAddr string) error {
  	if err := cl.listenSelfCheck(serverAddr); err != nil {
  		return err
  	}
  	return cl.challenge(serverAddr)
  }
docs: // Challenge requests a challenge from the server running at serverAddr, which
  // should be a host name or of the hostname:port form, and then fulfills that challenge.