public function setRequired($required)
    {
        if (!in_array(
            $required,
            [
                \Parable\Console\Parameter::PARAMETER_REQUIRED,
                \Parable\Console\Parameter::PARAMETER_OPTIONAL,
            ]
        )) {
            throw new \Parable\Console\Exception('Required must be one of the PARAMETER_* constants.');
        }
        $this->required = $required;
        return $this;
    }