clear the cart on add product

Hi,
I have a digital product that have 3 versions (with different prices)
I already configured the product with the vars.
I dont want to allow my customer to select quantity of product (like you did) so I removed it from the product page and the cart page.
The problem is, if a user decide to go back and add to cart again, he will have 2 quantity (same as if I click add Essential product, then go to the product page again and click add again, I will have 2 quantity), so I limited the quantity to 1 and the item quantity to 1, so only one product could be bought at the same time.
The problem is, if a user decide to go back and select a different version, and click on add to cart, the old version is still there.
I'm looking for a way to clear the cart every time an item is added to it, so the most resent item will be in the cart.

Thanks.

any one can help?

Thanks for not answering, learned a lot in the way....
here is the way to do it, be advised this is hard coding for the system so it may break on updates and such, always keep a change log for changes you make!!!
this is really simple, please , hika support, if this is not correct let me know.
you should edit this file: administrator/components/com_hikashop/classes/cart.php
just insert this code:

$this->resetCart(true);

function update($product_id,$quantity=1,$add=0,$type='product',$resetCartWhenUpdate=true,$force=false){

Thought occured to me that a solution using a plugin might be more flexible and of a wider interest.
Something like this at line 208 (not tried it - just a suggestion for further consideration).

JPluginHelper::importPlugin( 'hikashop' );
$dispatcher =& JDispatcher::getInstance();
if (!$dispatcher->trigger( 'onBeforeUpdateCart', $this, $cart, $product_id, $quantity, $add, $type )) return false;

Thanks for your help, I was hopping I can make a plugin that will not edit the core files...
Will look in to it (as I never built a plugin before) as soon as I have time....

I wish I know how to make plugins, think it would have made my life much easier.
Just wanted to let you know I have edited my code as it had a "bug" when I entered coupon on the cart was also cleared.
so i just added a check to my line:

if($type=='product')$this->resetCart(true);

Hi Nicolas,
I'm looking to build this plug-in now, can you help me do it with the fix i added at the end?
I have copied the history plugin and renamed it (and the files inside) to clear_cart.
the code on the xml file:

<files>
		<filename plugin="clear_cart">clear_cart.php</filename>
	</files>
	<params addpath="/components/com_hikashop/params">
	</params>

class plgHikashopClear extends JPlugin
{
	function plgHikashopClear(&$subject, $config){
		parent::__construct($subject, $config);
		if(!isset($this->params)){
			$plugin =& JPluginHelper::getPlugin('hikashop', 'clear_cart');
			jimport('joomla.html.parameter');
			$this->params = new JParameter( $plugin->params );
		}
    }
    function onBeforeCartUpdate(){
		if($type=='product')$this->resetCart(true);
return true;
    }
}

thanks, this is my files now:
in cart.php i have removed my code and inserted this one instead on line 208 just before the closing of this if: if($this->cart->cart_id){

JPluginHelper::importPlugin( 'hikashop' );
$dispatcher =& JDispatcher::getInstance();
if (!$dispatcher->trigger( 'onBeforeCartUpdate', $this, $cart, $product_id, $quantity, $add, $type )) return false;

<?php
/**
 * @package		HikaShop for Joomla!
 * @version		1.5.6
 * @author		hikashop.com
 * @copyright	(C) 2010-2012 HIKARI SOFTWARE. All rights reserved.
 * @license		GNU/GPLv3 http://www.gnu.org/licenses/gpl-3.0.html
 */
defined('_JEXEC') or die('Restricted access');
?>
<?php
class plgHikashopClear_cart extends JPlugin
{
	function plgHikashopClear_cart(&$subject, $config){
		parent::__construct($subject, $config);
		if(!isset($this->params)){
			$plugin =& JPluginHelper::getPlugin('hikashop', 'Clear_cart');
			jimport('joomla.html.parameter');
			$this->params = new JParameter( $plugin->params );
		}
    }
    function onBeforeCartUpdate(){
		if($type=='product')$this->resetCart(true);
		return true;
    }
}