private function convertParameterAggregate($var, $arguments)
    {
        // Treat as list.
        $var = (array) $var;

        if (!empty($arguments['recursive'])) {
            $var = iterator_to_array(
                new \RecursiveIteratorIterator(
                    new \RecursiveArrayIterator(
                        $var
                    )
                )
            );
        }

        if (!$var) {
            return 'NULL';
        }

        if (!empty($arguments['key'])) {
            $var = array_keys($var);
        } else {
            // Use values.
            $var = array_values($var);
        }

        if ($arguments['aggregate'] == 'set') {
            $this->addParameter(implode(',', $var));

            return '?';
        }

        $this->addParameters($var);

        return rtrim(str_repeat('?,', count($var)), ',');
    }