Magento: Checkout - etc - config.xml

<!--Path app\code\local\ABC\Checkout\etc\config.xml-->
<!--http://magento.stackexchange.com/questions/45132/adding-a-product-to-the-cart-via-querystring-using-sku-->
<?xml version="1.0" ?>
<config>
    <frontend>
        <routers>
            <checkout>
                <args>
                    <modules>
                        <abc_checkout before="Mage_Checkout">ABC_Checkout</abc_checkout>
                    </modules>
                </args>
            </checkout>
        </routers>
    </frontend>
   <global>
    <helpers>
        <checkout>
            <rewrite>
                <cart>ABC_Checkout_Helper_Cart</cart>
            </rewrite>
        </checkout>
    </helpers>
  </global>
</config>