group discount not applied to one group of products

[infinity]

Hi,
I have clients group X and discount 40% applied to them.

Apart from it, I have group of stuff 'SUPER DISCOUNT'.

I want to make something like this:

clients group X have discount 40% to all groups off stuf BUT NOT TO 'SUPER DISCOUNT' - for this group they have 0% discount.

Id of group 'SUPER DISCOUNT' is 12


What file and what line should I modified to achieve this?

I'm using prestashop 1.3.3.0.

regards!