public function addKeyword($keyword, $typeClassName)
    {
        if (!class_exists($typeClassName)) {
            throw new \InvalidArgumentException(
                'The Value Object that needs to be created with a keyword "' . $keyword . '" must be an existing class'
                . ' but we could not find the class ' . $typeClassName
            );
        }

        if (!in_array(Type::class, class_implements($typeClassName), true)) {
            throw new \InvalidArgumentException(
                'The class "' . $typeClassName . '" must implement the interface "phpDocumentor\Reflection\Type"'
            );
        }

        $this->keywords[$keyword] = $typeClassName;
    }