Multiple attributes in single Excel cell

Question

I have been trying to find an answer to this question but miserably failed to do so. Accordingly, I am asking the hive mind. I am currently engaged in a research project where I look at how certain decision makers used certain authorities and, most importantly, how they used them.

Since I also do social network analysis, I tend to prefer using a spreadsheet. (I could use qualitative analysis software, but it does not work all that well.) My spreadsheet looks like this:

 

Citing decision | Cited authority | Topic | Cited to say what

Decision 1      | Authority 2     | Topic1| Cited to say xyz

 

However, I would like it to do this too (note the "Topic" column):

 

Citing decision | Cited authority | Topic         | Cited to say what

Decision 1      | Authority 2     | Topic1,Topic2 | Cited to say xyz

 

Now, this would mean that, if an authority has been invoked for more than one purpose, this should be somehow readable / filterable. It would be ideal if it could also work with Excel's autocomplete.

Should it be of any relevance, I am on a Mac.