Looks like e3f341f3 broke it, but we
wouldn't have noticed because the test isn't run in validate.

I think the change is OK.  We don't get a tick for x' any more, because
it is converted directly to a case instead of a let-binding, but we
still get a tick for the "pattern binding" (!x').  This could be better
- f.(...) isn't all that intuitive - but it's not strictly wrong.