Magento1 Stock Model Rewrite to add events

Stock.php

<?php
/**
 * Add events to observe stock qty change
 * 
 * @author Fabian Schmengler
 *
 */
class SGH_ShippingExpress_Model_CatalogInventory_Stock
	extends Mage_CatalogInventory_Model_Stock
{
	const EVENT_CORRECT_STOCK_ITEMS_QTY_BEFORE = 'cataloginventory_stock_item_correct_qty_before';
	const EVENT_CORRECT_STOCK_ITEMS_QTY_AFTER = 'cataloginventory_stock_item_correct_qty_after';
	
	/**
	 * (non-PHPdoc)
	 * @see Mage_CatalogInventory_Model_Stock::registerProductsSale()
	 */
    public function registerProductsSale($items)
    {
    	Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_BEFORE, array(
    		'stock'     => $this,
    		'items_obj' => (object)array('items' => &$items),
    		'operator'  => '+'
    	));
    	$result = parent::registerProductsSale($items);
    	Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_AFTER, array(
	    	'stock'          => $this,
	    	'items'          => $items,
	    	'fullsave_items' => $result,
    		'operator'       => '+'
    	));
    	return $result;
    }
    /**
     * (non-PHPdoc)
     * @see Mage_CatalogInventory_Model_Stock::revertProductsSale()
     */
    public function revertProductsSale($items)
    {
    	Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_BEFORE, array(
	    	'stock'     => $this,
	    	'items_obj' => (object)array('items' => &$items),
	    	'operator'  => '-'
    	));
    	$result = parent::revertProductsSale($items);
    	Mage::dispatchEvent(self::EVENT_CORRECT_STOCK_ITEMS_QTY_BEFORE, array(
    		'stock'          => $this,
    		'items'          => $items,
    		'fullsave_items' => $result,
    		'operator'       => '-'
    	));
    	return $result;
    }
}