code: func (c *client) Add(s interface{}, cert *ssh.Certificate, comment string) error {
  	if cert == nil {
  		return c.insertKey(s, comment)
  	} else {
  		return c.insertCert(s, cert, comment)
  	}
  }
docs: // Insert adds a private key to the agent. If a certificate is given,
  // that certificate is added instead as public key.