$increment is (or used to be) unique in that it was the only function that changed its argument. It reminded me of learning about "destructive functions" in LISP a long time ago. No matter where/how you use it, it increments. So "set a = $increment(a)" is redundant, but still OK to use. As you saw, developers started to use "if $increment(a)" alone, because that was shorter but also works fine, even thought it looks strange. I wasn't aware of "do $increment(a)" being allowed, which does look nicer, so I also learned something new from this thread, thanks.

And let's not forget to mention the popular and useful "set a($increment(a)) =  something"