anchor.diff:  { configuration ? import ./lib/from-env.nix "NIXOS_CONFIG" <nixos-config>
  -, system ? builtins.currentSystem
  +, targetSystem ? builtins.currentSystem
  +, hostSystem ? builtins.currentSystem
   , extraModules ? []
     # This attribute is used to specify a different nixos version, a different
     # system or additional modules which might be set conditionally.
  
positive.code:   };
  
    eval = import ./lib/eval-config.nix {
      inherit system;
      modules = [ configuration reEnterModule ] ++ extraModules;
    };
  
  
negative.code: 
    # This is for `nixos-rebuild build-vm'.
    vmConfig = (import ./lib/eval-config.nix {
      inherit targetSystem hostSystem;
      modules = [ configuration reEnterModule ./modules/virtualisation/qemu-vm.nix ] ++ extraModules;
    }).config;
  
    # This is for `nixos-rebuild build-vm-with-bootloader'.
    vmWithBootLoaderConfig = (import ./lib/eval-config.nix {
      inherit targetSystem hostSystem;
      modules =
        [ configuration reEnterModule
          ./modules/virtualisation/qemu-vm.nix
  