[Nix-dev] Mark two packages incompatible, but with the same priority?

[Jeffrey David Johnson]

I'm trying to add BitcoinXT support. Changes here:

https://github.com/NixOS/nixpkgs/compare/master...jefdaj:bitcoinxt-pullreq

It works as long as you only install bitcoin or bitcoin-xt, but trying
one after the other they collide. Is there a way to set it up so
installing either one overrides the other cleanly?