anchor.diff:        example = "0.8";
         description = ''
           Opacity of dropdown and popup menu.
  +      '';
  +    };
  +
  +    opacityRules = mkOption {
  +      type = types.listOf types.str;
  +      default = [];
  +      description = ''
  +        Opacity rules to be handled by compton.
         '';
       };
   
  
positive.code:       active-opacity   = ${cfg.activeOpacity};
        inactive-opacity = ${cfg.inactiveOpacity};
        menu-opacity     = ${cfg.menuOpacity};
        
        # other options
        backend = ${toJSON cfg.backend};
        vsync = ${toJSON cfg.vSync};
  
negative.code: 
    cfg = config.services.compton;
  
    configFile = let
      opacityRules = optionalString (length cfg.opacityRules != 0)
        (concatStringsSep "\n"
          (map (a: "opacity-rule = [ \"${a}\" ];") cfg.opacityRules)
        );
    in pkgs.writeText "compton.conf"
      (optionalString cfg.fade ''
        # fading
        fading = true;
  