anchor.diff:        pythonAttrs = lib.filterAttrs (name: _: isPython name);
   
       in {
  -      overlay = final: prev:
  +      overlays.default = final: prev:
           {
             bifrost = final.callPackage bifrost { };
             bifrost-doc = final.callPackage bifrost-doc { };
  
positive.code:           do (import nixpkgs {
              inherit system;
              config.allowUnfree = true;
              overlays = [ self.overlay ];
            }));
  
        # Which python3 packages should be modified by the overlay?
  
negative.code: 
          in { inherit (pkgs) bifrost-doc; } // cgens // bfs // pys);
  
        devShells = eachSystem (pkgs: {
          default = let
            pre-commit = pre-commit-hooks.lib.${pkgs.system}.run {
              src = ./.;
              hooks.nixfmt.enable = true;
  