If I still may: roll-back and reproducible configuration was already proposed in the past? The idea raised by Nix devs was the a purely functional approach was a way to implement it. Of course people can have similar ideas with other techniques. If it a very practical question because I'm sure all Arch users in some point or another had to do a roll-back after a complex system update, and then they find themselves in a difficult situation to figure out how to revert all those changes. Pro Audio users, for instance, might want to have their system configuration in a state just before the change that broke lv2 support on Ardour. Nix approach may be not the only one, but their ideas let people see the difference between same packages build with different libs, or know to set a exact system configuration more easily.
