public function toSlug(string $var, bool $strict = true): string
    {
        $slug = $var; // Initialize.
        $slug = mb_strtolower($this->c::forceAscii($slug));
        $slug = preg_replace('/[^a-z0-9]+/u', '-', $slug);
        $slug = $this->c::mbTrim($slug, '', '-');

        if ($strict && $slug && !preg_match('/^[a-z]/u', $slug)) {
            $slug = 'x'.$slug; // Force `^[a-z]`.
        }
        return $slug;
    }