public static function extensionMethod($name, $callback = null)
{
    // ...
    if ($callback === null) {
        return Nette\Utils\ObjectMixin::getExtensionMethod($class, $name);
    } else {
        Nette\Utils\ObjectMixin::setExtensionMethod($class, $name, $callback);
    }
}

public static function extensionMethod($name, $callback = null)
{
    // ...
    if ($callback === null) {
        return Nette\Utils\ObjectMixin::getExtensionMethod($class, $name);
    }

    Nette\Utils\ObjectMixin::setExtensionMethod($class, $name, $callback);
}