尝试覆盖模型但不使用magento

Try to override model of Mage_Paypal_Model_Express_Checkout but not working.

I try to override start() in "Mage_Paypal_Model_Express_Checkout" in magento.

The bellow code used but not working.

File : code/local/Live/Paypal/Model/Express/Checkout.php

class Live_Paypal_Model_Express_Checkout extends 
Mage_Paypal_Model_Express_Checkout 
{

    public function start($returnUrl, $cancelUrl, $button = null)
    {
        echo "fasdfasdf";exit;
    }
}

File : code/local/Live/Paypal/etc/config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Live_Paypal>
            <version>1.0.0</version>
        </Live_Paypal>
    </modules>
    <global>
        <models>
            <paypal>
                <rewrite>
                  <express_checkout>Live_Paypal_Model_Express_Checkout</express_checkout>
                </rewrite>
            </paypal>
        </models>
    </global>
</config>

File : app/etc/modules/Live_Paypal.xml

<?xml version="1.0"?>
<config>
    <modules>
        <Live_Paypal>
            <active>true</active>
            <codePool>local</codePool>
        </Live_Paypal>
    </modules>
</config>