Writing a formula that concatenates non checked items from a

Writing a formula that concatenates non checked items from a

I've got a multi-select checkbox field named Category, and I want to create a field named Categories that concatenates the non-checked items from Category. If I just do a lookup, it only offers me list fieldtypes, and if I enter formula, I can't for the life of me figure out how to extract the non-blank values. (I downloaded the Deluge Script Help Document but couldn't find anything in there either.)

Am I just out of luck?

Thanx
zoho