How to add new argument or parameter in _applicable method of class account_tax of account module??

I know that in openerp taxes are calculating on the basis of unit price and its quantity but i want it to calculate on unit price ,its quantity and one more parameter (such as shipping cost ).How can i achieve that. I go through the methods of account module and come to know that _applicable function is the base function to calculate tax. Can some one tell me how to override this function and add a new parameter like shipping_cost in it. ThankYou in advance