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

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