This looks interesting, and definitely useful for non-C/Python languages, which existing reducers I know of mostly don't have explicit support for! I can't get it to build though (I've filed an issue).

I was also wondering: is there a UI while reduction happens? I've found Shrink Ray's UI improvements in the last year to be much more useful than I first expected: not just because it gives me something to look at, but because it really helps me understand if reduction is on the right path or not. [Some of the new Shrink Ray extras like being able to rewind reduction to a past point and to skip passes are also really useful too.]