public static function change_case($st, $case = 'x'): string
  {
    $st = self::cast($st);
    $case = substr(strtolower((string)$case), 0, 1);
    switch ( $case ){
      case "l":
        $case = MB_CASE_LOWER;
        break;
      case "u":
        $case = MB_CASE_UPPER;
        break;
      default:
        $case = MB_CASE_TITLE;
    }
    if ( !empty($st) ){
      $st = mb_convert_case($st, $case);
    }
    return $st;
  }