I should have been more clear.

I use a water bath and TF4 fixer for film, always. The stop is for Azo, because a water bath would continue the development too long. So it's just for paper. And, if it matters..I use plain thiosulfate and bisulfite and then plain thio to fix the paper.

But after reading the interesting responses, I do have a question...isn't citric acid a developer, also? If you use it as a developer AND as a stop, how does the medium (film in that case, I guess) know when to stop developing???

I would be interested in making some rapid odorless top myself, if anyone has a good formula.