OCaml PPX - What's the Process Like?

OCaml has this thing called PPX, which essentially allows you to extend OCaml syntax. And so the particular PPX that I got to write was one that made it a lot easier to use like a particular commonly used idiom in Incredom. So what was the process like of working on this more compiler-like part of the system? It was actually surprisingly smooth, there's some very good libraries out there that make it very easy to do transformations on OCaml's syntax. But there were definitely some tricky things like getting the source code positions to match up properly between the old tree and the new tree and stuff like that.