I am trying to modify coupon code functionality. I have got a situation where I am not able to find where a coupon code is validated. I have searched the entire code base. Can anyone point me in right direction?

I am trying to modify coupon code functionality. I have got a situation where I am not able to find where a coupon code is validated. I have searched the entire code base. Can anyone point me in right direction?

I am trying to apply Multiple Coupon codes.