anchor.diff:  
   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
  
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: 
          script =
            ''
              # Ensure that this job is restarted when fstab changed:
              # ${fstab}
              exec > /dev/console 2>&1
              echo "mounting filesystems..."
              ${pkgs.mountall}/sbin/mountall
  