public function usage()
    {
        $name = (strlen($this->_short) > 0) ? ('-' . $this->_short) : ('--' . $this->_name);
        $default = '';
        if (strlen($this->_default) > 0 && $this->_default !== true) {
            $default = ' ' . $this->_default;
        }
        if ($this->_choices) {
            $default = ' ' . implode('|', $this->_choices);
        }

        return sprintf('[%s%s]', $name, $default);
    }