public function hasKeyword($version, $keyword)
    {
        $cache = & $this->keywordsCache[$version];

        if ($cache === null) {
            $cache = [];

            foreach ($this->getConstraints($version) as $constraint) {
                foreach ($constraint->keywords() as $constraintKeyword) {
                    $cache[$constraintKeyword] = true;
                }
            }
        }

        return isset($cache[$keyword]);
    }