Oh, cool. I don't see it being used anywhere though, apart from being available as a system activation function?

In Guix grafts are used rather extensively, mainly for security updates, but also command-line transformations such as "--with-debug-info".

It wouldn't be a nice interface for packagers to use.

More importantly, it's not enough to implement grafts because it's not recursive, as discussed at https://guix.gnu.org/en/blog/2016/timely-delivery-of-securit... .