public function isImplemented($pFunction)
    {
        $pFunction = strtoupper($pFunction);
        $notImplemented = !isset(self::$phpSpreadsheetFunctions[$pFunction]) || (is_array(self::$phpSpreadsheetFunctions[$pFunction]['functionCall']) && self::$phpSpreadsheetFunctions[$pFunction]['functionCall'][1] === 'DUMMY');

        return !$notImplemented;
    }