Donot mesh previous session magento with current cart

If you want that the current cart items not to mesh up with previous session when you log on , replace your
app/code/core/Mage/Checkout/Model/observer.php file code with this one and you are done.
What we wanted was to prevent the cart from the old session from merging into the cart of the current session at the point when thecustomer login
<?php
/**
 * Magento
 *
 * NOTICE OF LICENSE
 *
 * This source file is subject to the Open Software License (OSL 3.0)
 * that is bundled with this package in the file LICENSE.txt.
 * It is also available through the world-wide-web at this URL:
 * http://opensource.org/licenses/osl-3.0.php
 * If you did not receive a copy of the license and are unable to
 * obtain it through the world-wide-web, please send an email
 * to license@magentocommerce.com so we can send you a copy immediately.
 *
 * DISCLAIMER
 *
 * Do not edit or add to this file if you wish to upgrade Magento to newer
 * versions in the future. If you wish to customize Magento for your
 * needs please refer to http://www.magentocommerce.com for more information.
 *
 * @category   Mage
 * @package    Mage_Checkout
 * @copyright  Copyright (c) 2008 Irubin Consulting Inc. DBA Varien (http://www.varien.com)
 * @license    http://opensource.org/licenses/osl-3.0.php  Open Software License (OSL 3.0)
 */

/**
 * Checkout observer model
 *
 * @category   Mage
 * @package    Mage_Checkout
 * @author      Magento Core Team <core@magentocommerce.com>
 */
class Mage_Checkout_Model_Observer
{
    public function unsetAll()
    {
        Mage::getSingleton('checkout/session')->unsetAll();
    }

    public function loadCustomerQuote()
    {
        $lastQid = Mage::getSingleton('checkout/session')->getQuoteId(); //quote id during session before login;
        if ($lastQid) { //before login session exists means cart has items                           
            $customerQuote = Mage::getModel('sales/quote')
                ->loadByCustomer(Mage::getSingleton('customer/session')->getCustomerId()); //the cart from last login        
            //set it to the session before login and remove its items if any
            $customerQuote->setQuoteId($lastQid);
            $this->_removeAllItems($customerQuote);
           
        } else { //no session before login, so empty the cart (current cart is the old cart)
            $quote = Mage::getModel('checkout/session')->getQuote();                               
            $this->_removeAllItems($quote);
        }
    }

    protected function _removeAllItems($quote){
        //reset all custom attributes in the quote object here, eg:    
       // $quote->setDestinationCity('');
       
        foreach ($quote->getAllItems() as $item) {
            $item->isDeleted(true);
            if ($item->getHasChildren()) foreach ($item->getChildren() as $child) $child->isDeleted(true);
        }
        $quote->collectTotals()->save();       
    } //_removeAllItems
   
   
    public function salesQuoteSaveAfter($observer)
    {
        $quote = $observer->getEvent()->getQuote();
        /* @var $quote Mage_Sales_Model_Quote */
        if ($quote->getIsCheckoutCart()) {
            Mage::getSingleton('checkout/session')->getQuoteId($quote->getId());
        }
    }
}