> Should we just swap the order and byte prefix of this and SigShutdown?

Nah, it's fine as it is. If we were going to swap the byte prefix, it would make sense IMHO to move `SigShutdown` to 1 instead. If we didn't need to support GHC 7.4.*, it would be even better to just derive-generic here, but I've not been able to make that work consistently with older releases (despite generic support being in place earlier than 7.4), on account of crazy dependencies.