code: func NewSSHAgentAuth(u string) (*PublicKeysCallback, error) {
  	var err error
  	if u == "" {
  		u, err = username()
  		if err != nil {
  			return nil, err
  		}
  	}
  
  	a, _, err := sshagent.New()
  	if err != nil {
  		return nil, fmt.Errorf("error creating SSH agent: %q", err)
  	}
  
  	return &PublicKeysCallback{
  		User:     u,
  		Callback: a.Signers,
  	}, nil
  }
docs: // NewSSHAgentAuth returns a PublicKeysCallback based on a SSH agent, it opens
  // a pipe with the SSH agent and uses the pipe as the implementer of the public
  // key callback function.