It's unclear what role "sometool" is supposed to be playing

[dabrahams]

In this section it's not clear what "sometool" is supposed to be. I'm… guessing… it's an executable target that is used in a build step? IMO it would be helpful if this guide were phrased in terms of the actual commands invoked. It would be even more helpful if there was a tiny sample project that, say, preprocesses a .swiftx file into a .swift file with the identity transformation, or something.

[dabrahams]

I guess I begin to see that explained in the next section… but it would be nice if it weren't left a mystery until later. A little set-up would help… you know, "say you're trying to ___________”