private function removeWhitespace(): void
    {
        $string = preg_replace('/ {2,}/', Tokens::SPACE, $this->normalisedString);
        $lines = [];

        foreach (explode(Tokens::LINE_FEED, $string) as $line) {
            if ('' !== $line = trim($line)) {
                $lines[] = $line;
            }
        }
        $this->normalisedString = implode(Tokens::LINE_FEED, $lines);
    }