mytheme_links__system_main_menu() doesn't work as expected

I use the starterkit as a child of zen and can see in the page.tpl.php that primary links are called via theme(array('links__system_main_menu', 'links'),..., and if I create a function called mytheme_links__system_main_menu() in my subtheme's template.php, this hook is not register and won't work, so it falls back to zen_links().