anchor.diff:        options = mkOption {
           default = "";
           example = "--max-freed $((64 * 1024**3))";
  +        type = types.string;
           description = "
             Options given to <filename>nix-collect-garbage</filename> when the
             garbage collector is run automatically.
  
positive.code: 
        automatic = mkOption {
          default = false;
          example = true;
          description = "
            Automatically run the garbage collector at specified dates.
          ";
  
negative.code: 
        dates = mkOption {
          default = "15 03 * * *";
          type = types.string;
          description = "
            Run the garbage collector at specified dates to avoid full
            hard-drives.
  