public function addWord(string $capitalCase, string $lowerCase = null)
    {
        if ($lowerCase == null) {
            $lowerCase = strtolower($capitalCase);
        }

        $index = strtolower($lowerCase);
        $record = [$capitalCase, $lowerCase];

        $this->words[$index] = $record;

        if ($index !== strtolower($capitalCase)) {
            $this->words[strtolower($capitalCase)] = $record;
        }
    }