{lib, ...}: let
  firstNonNull = xs: lib.findFirst (x: x != null) "unknown" xs;
in {
  gitRevOrLastModified = flake:
    firstNonNull [
      (lib.mapNullable (r: "git-" + r) (flake.shortRev or flake.rev or null))
      (lib.mapNullable (n: "from-" + (lib.toLower n)) flake.sourceInfo.lastModifiedDate or null)
    ];
}