The implementation looks for ‘graft’ fields in all the transitive inputs
of the package being built.

Currently it works but needs more testing and documentation.  There are
also performance issues that need to be worked out.

Comments welcome!

The general idea was implemented by Shea Levy in Nixpkgs¹, and also
suggested by Mark Weaver on IRC, who helped refine how things should
work in Guix–thanks!

Ludo’.

¹ https://github.com/NixOS/nixpkgs/commit/d1662d715514e6ef9d3dc29f132f1b3d8e608a18