public function toPlural($word)
    {
        // Try to get the cached plural form from the singular.
        $cache = $this->getCachedPlural($word);

        if ($cache !== false) {
            return $cache;
        }

        // Check if the word is a known singular.
        if ($this->getCachedSingular($word)) {
            return false;
        }

        // Compute the inflection.
        $inflected = $this->matchRegexRule($word, 'plural');

        if ($inflected !== false) {
            $this->setCache($word, $inflected);

            return $inflected;
        }

        return false;
    }