anchor.diff:      listToAttrs (map (attr: { name = attr; value = f attr set.${attr}; }) (attrNames set))
     );
   
  +  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/lists.nix#L295
  +  range = first: last: if first > last then [] else builtins.genList (n: first + n) (last - first + 1);
  +
  +  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L257
  +  stringToCharacters = s: map (p: builtins.substring p 1 s) (range 0 (builtins.stringLength s - 1));
  +
  +  # https://github.com/NixOS/nixpkgs/blob/0258808f5744ca980b9a1f24fe0b1e6f0fecee9c/lib/strings.nix#L269
  +  stringAsChars = f: s: concatStrings (map f (stringToCharacters s));
  +  concatMapStrings = f: list: concatStrings (map f list);
  +  concatStrings = builtins.concatStringsSep "";
  +
  +  # https://github.com/NixOS/nixpkgs/blob/8a9f58a375c401b96da862d969f66429def1d118/lib/attrsets.nix#L331
  +  optionalAttrs = cond: as: if cond then as else {};
  +
     # fetchTarball version that is compatible between all the versions of Nix
  -  builtins_fetchTarball = { url, sha256 }@attrs:
  +  builtins_fetchTarball = { url, name ? null, sha256 }@attrs:
       let
         inherit (builtins) lessThan nixVersion fetchTarball;
       in
         if lessThan nixVersion "1.12" then
  -        fetchTarball { inherit url; }
  +        fetchTarball ({ inherit url; } // (optionalAttrs (!isNull name) { inherit name; }))
         else
           fetchTarball attrs;
   
     # fetchurl version that is compatible between all the versions of Nix
  -  builtins_fetchurl = { url, sha256 }@attrs:
  +  builtins_fetchurl = { url, name ? null, sha256 }@attrs:
       let
         inherit (builtins) lessThan nixVersion fetchurl;
       in
         if lessThan nixVersion "1.12" then
  -        fetchurl { inherit url; }
  +        fetchurl ({ inherit url; } // (optionalAttrs (!isNull name) { inherit name; }))
         else
           fetchurl attrs;
   
  
positive.code: 
      if ! builtins.hasAttr "type" spec then
        abort "ERROR: niv spec ${name} does not have a 'type' attribute"
      else if spec.type == "file" then fetch_file pkgs spec
      else if spec.type == "tarball" then fetch_tarball pkgs spec
      else if spec.type == "git" then fetch_git spec
      else if spec.type == "builtin-tarball" then fetch_builtin-tarball spec
      else if spec.type == "builtin-url" then fetch_builtin-url spec
      else
        abort "ERROR: niv spec ${name} has unknown type ${builtins.toJSON spec.type}";
  
    # Ports of functions for older nix versions
  
  
negative.code:   # The fetchers. fetch_<type> fetches specs of type <type>.
    #
  
    fetch_file = pkgs: name: spec:
      let
        name' = sanitizeName name + "-src";
      in
        if spec.builtin or true then
          builtins_fetchurl { inherit (spec) url sha256; name = name'; }
        else
          pkgs.fetchurl { inherit (spec) url sha256; name = name'; };
  
    fetch_tarball = pkgs: name: spec:
      let
        name' = sanitizeName name + "-src";
      in
        if spec.builtin or true then
          builtins_fetchTarball { name = name'; inherit (spec) url sha256; }
        else
          pkgs.fetchzip { name = name'; inherit (spec) url sha256; };
  
    fetch_git = name: spec:
      let
        ref =
          if spec ? ref then spec.ref else
            if spec ? branch then "refs/heads/${spec.branch}" else
              if spec ? tag then "refs/tags/${spec.tag}" else
                abort "In git source '${name}': Please specify `ref`, `tag` or `branch`!";
      in
        builtins.fetchGit { url = spec.repo; inherit (spec) rev; inherit ref; };
  
    fetch_local = spec: spec.path;
  
    fetch_builtin-tarball = name: throw
      ''[${name}] The niv type "builtin-tarball" is deprecated. You should instead use `builtin = true`.
          $ niv modify ${name} -a type=tarball -a builtin=true'';
  
    fetch_builtin-url = name: throw
      ''[${name}] The niv type "builtin-url" will soon be deprecated. You should instead use `builtin = true`.
          $ niv modify ${name} -a type=file -a builtin=true'';
  
    #
    # Various helpers
    #
  
    # https://github.com/NixOS/nixpkgs/pull/83241/files#diff-c6f540a4f3bfa4b0e8b6bafd4cd54e8bR695
    sanitizeName = name:
      (
        concatMapStrings (s: if builtins.isList s then "-" else s)
          (
            builtins.split "[^[:alnum:]+._?=-]+"
              ((x: builtins.elemAt (builtins.match "\\.*(.*)" x) 0) name)
          )
      );
  
    # The set of packages used when specs are fetched using non-builtins.
    mkPkgs = sources: system:
      let
        sourcesNixpkgs =
          import (builtins_fetchTarball { inherit (sources.nixpkgs) url sha256; }) { inherit system; };
        hasNixpkgsPath = builtins.any (x: x.prefix == "nixpkgs") builtins.nixPath;
        hasThisAsNixpkgsPath = <nixpkgs> == ./.;
      in
  