code: func (ftp *FTP) cmd(expects string, command string, args ...interface{}) (line string, err error) {
  	if err = ftp.send(command, args...); err != nil {
  		return
  	}
  
  	if line, err = ftp.receive(); err != nil {
  		return
  	}
  
  	if !strings.HasPrefix(line, expects) {
  		err = errors.New(line)
  		return
  	}
  
  	return
  }
docs: // private function to send command and compare return code with expects