anchor.diff:  
     ###### implementation
   
  -  config = mkIf config.programs.file-roller.enable {
  +  config = mkIf cfg.enable {
   
  -    environment.systemPackages = [ pkgs.gnome.file-roller ];
  +    environment.systemPackages = [ cfg.package ];
   
  -    services.dbus.packages = [ pkgs.gnome.file-roller ];
  +    services.dbus.packages = [ cfg.package ];
   
     };
   
  
positive.code: 
  with lib;
  
  {
  
    # Added 2019-08-09
    imports = [
  
negative.code: 
        enable = mkEnableOption "File Roller, an archive manager for GNOME";
  
        package = mkOption {
          type = types.package;
          default = pkgs.gnome.file-roller;
          defaultText = literalExpression "pkgs.gnome.file-roller";
          description = "File Roller derivation to use.";
        };
  
      };
  
    };
  