[Haskell-cafe] generics-sop equivalent of everywhere/mkT?

Before I cut down my code to a test case, are there any examples of a generics-sop equivalent of syb's everywhere/mkT? What's the way to operate on a product, replace the "I" argument with a compatible argument and walk back through the isomorphism, i.e. "to $ <something> $ from". I'm hacking on a Z80 system emulator (TRS-80 Model I system, more specifically). There are a couple of places where it'd be smoother in the disassembler to transform the disassembled instruction sequence (converting addresses to labels) before output. Consequently, cutting down code to an example is a bit painful -- examples would help. -scooter