anchor.diff:  
           script =
             ''
  +            # Ensure that this job is restarted when fstab changed:
  +            # ${fstab}
               exec > /dev/console 2>&1
               echo "mounting filesystems..."
               ${pkgs.mountall}/sbin/mountall
  
positive.code:       ++ config.system.fsPackages;
  
      environment.etc = singleton
        { source = pkgs.writeText "fstab"
            ''
              # This is a generated file.  Do not edit!
  
              # Filesystems.
              ${flip concatMapStrings config.fileSystems (fs:
                  (if fs.device != null then fs.device else "/dev/disk/by-label/${fs.label}")
                  + " " + fs.mountPoint
                  + " " + fs.fsType
                  + " " + fs.options
                  + " 0"
                  + " " + (if fs.fsType == "none" || fs.noCheck then "0" else
                           if fs.mountPoint == "/" then "1" else "2")
                  + "\n"
              )}
  
              # Swap devices.
              ${flip concatMapStrings config.swapDevices (sw:
                   "${sw.device} none swap\n"
              )}
            '';
          target = "fstab";
        };
  
  
negative.code: 
  with pkgs.lib;
  
  let
  
    fstab = pkgs.writeText "fstab"
      ''
        # This is a generated file.  Do not edit!
  
        # Filesystems.
        ${flip concatMapStrings config.fileSystems (fs:
            (if fs.device != null then fs.device else "/dev/disk/by-label/${fs.label}")
            + " " + fs.mountPoint
            + " " + fs.fsType
            + " " + fs.options
            + " 0"
            + " " + (if fs.fsType == "none" || fs.noCheck then "0" else
                     if fs.mountPoint == "/" then "1" else "2")
            + "\n"
        )}
  
        # Swap devices.
        ${flip concatMapStrings config.swapDevices (sw:
             "${sw.device} none swap\n"
        )}
      '';
  
  in
      
  {
  
    ###### interface
  