I needed it because the integers wer accumulated back-to-front. I wanted to then write them back, one-by-one in reverse. But thanks to @zkamsler's [suggestion](https://github.com/mozilla/rust/pull/12382/files#r9853168) we can now switch this back! I would like to change the order to `positive`, `prefix`, `buf`, as that reflects the order that those things will be printed.