How to exclude customer groups in a calculated field?

Hello,

I'm new to SugarCRM and its developing tools. 

I've built a calculated field in the opportunities module (sugar 8.0 Professional) and it works fine. 

ifElse(greaterThan($amount,10000),divide($amount,3),0)

--> installment payments should be divided by 3. if they exceed 10,000€.

However, this should not be done for certain customer groups. After googling for while, I couldn't find a function which excludes groups or the functions I've tried didn't work.

The name of the field is $icc_customer_group and customer groups are for example "university". 

I hope one of you can help me. 

Thanks!