List all categories in one cell

Summary

How can I use a report template to put all the categories in one cell?

Content

I am trying to create a report that generates a list of courses and some information about them, including categories they're mapped to.  Right now it looks like this:

Course 1	Description of course 1	Category 1
Course 1	Description of course 1	Category 2
Course 2	Description of course 2	Category 1
Course 2	Description of course 2	Category 3
Course 2	Description of course 2	Category 4

I am trying to make it look like this:

Course 1	Description of course 1	Category 1, Category 2
Course 2	Description of course 2	Category 1, Category 3, Category 4

I am trying to use a "repeating group" inside the third cell which will loop through the categories, but BI Publisher won't let me put a repeating group inside another repeating group.