{ nixpkgsPath ? <nixpkgs> }:

self: super: {
  selfExpr = let
    config = builtins.removeAttrs self.config [ "_undeclared" ];
    configJson = builtins.toJSON config;
  in
    self.writeTextFile {
    name = "nixpkgs-self";
    destination = "/default.nix";
    text = ''
      { ... } @ args:

      import ${builtins.toString nixpkgsPath} {
        config = (builtins.fromJSON '''
          ${configJson}
        ''') // args.config or {};
      } // builtins.removeAttrs args [ "config" ]
    '';
  };
}