let sources = import ./nix/sources.nix;in { system ? builtins.currentSystem   , nixpkgs ? sources.nixpkgs   , multiversal_src ? if builtins.pathExists ./multiversal/make-multiverse.rb then       ./multiversal     else       sources.multiversal   , ...   }: let  lib = ((import nixpkgs) { inherit system; }).lib;   retroPlatforms = (import nix/platforms.nix) { inherit lib; };    # A Nixpkgs overlay.  overlay = lib.composeManyExtensions [    ((import nix/overlay.nix) { inherit multiversal_src; })    (import nix/universal.nix)    (import nix/samples.nix)  ];   overlaidPkgs = import nixpkgs {    inherit system;    overlays = [ overlay ];  };   crossPkgs = lib.mapAttrs    (name: plat:      import nixpkgs {        inherit system;        overlays = [ overlay ];        crossSystem = plat;        config = { allowUnsupportedSystem = true; };      })    retroPlatforms;  targetPkgs = lib.mapAttrs (name: cross: cross.buildPackages) crossPkgs;   shell = lib.mapAttrs    (name: cross:      cross.mkShell        {          nativeBuildInputs = with overlaidPkgs; [            retro68.hfsutils            retro68.tools            cmake            gnumake          ];          buildInputs = [ cross.retro68.console ];        } // cross)    crossPkgs; inbuiltins.trace  "Warning: Retro68's default.nix is deprecated and will disappear soon. Please use the flake instead."  (shell.m68k // shell // {    inherit overlay;    inherit (overlaidPkgs) retro68;    targetPkg = targetPkgs;  })