How is customer_logged_in/out handle added to Magento layout?

I was wondering how Magento merge or handle into handle in layout.

I was expecting something like

<default>
    <update handle="customer_logged_in" if="logged_in"/>
    <update handle="customer_logged_out" if="logged_out"/>
</default>

Obviously, there is not such codes in layouts.

I searched within Magento folder for keyword “customer_logged_in”, but only occurrences I could find in layout/customer.xml.

So, how is customer_logged_in/out handle added to Magento layout?

At last, I found

class Mage_Customer_Model_Observer
{
    public function beforeLoadLayout($observer)
    {
        $loggedIn = Mage::getSingleton('customer/session')->isLoggedIn();

        $observer->getEvent()->getLayout()->getUpdate()
           ->addHandle('customer_logged_'.($loggedIn?'in':'out'));
    }
}

by tracing addHandle() in app/core/Mage/Core/Model/Layout/Update.php.

I must say using observer adding layout handle makes very easy for other modules to detect customer login status.