> I feel like the correct answer the substantial majority of the time is "comma separated".

Right. But, I don't. And the fact that we disagree about this tells me that it's the exact reason why the type should be specified. While I agree that if you're returning a list that is not going to be output to the stylesheet, the type doesn't matter, but if it is going to be output, there's no obvious default.

I have changed this function to the signature you suggested so that it is easier to select the type at runtime -- this is a very common pattern needed for writing list functions that accept either kind of list. However, I am still keeping the type argument required.