Oooh -- good idea. I don't think I have an instruction that quite matches. Retroputer's branch instructions are directly tied to the flags, so you can do `br !c(arry)`, but that doesn't rule out zero like `ja` does. At a quick glance I don't think that should rule something like this out, though. :-)

That looks like a really interesting book -- I think I shall add it to my reading list!