anchor.diff:          # check if the path is a directory or a file
           if isDir path then
             # it's a directory, so the set of overlays from the directory, ordered lexicographically
  -          let content = readDir path; in
  +          let content = builtins.readDir path; in
             map (n: import (path + ("/" + n)))
  -            (builtins.filter (n: builtins.match ".*\\.nix" n != null || pathExists (path + ("/" + n + "/default.nix")))
  -              (attrNames content))
  +            (builtins.filter (n: builtins.match ".*\\.nix" n != null || builtins.pathExists (path + ("/" + n + "/default.nix")))
  +              (builtins.attrNames content))
           else
             # it's a file, so the result is the contents of the file itself
             import path;
       in
  -      if pathOverlays != "" && pathExists pathOverlays then overlays pathOverlays
  -      else if pathExists homeOverlaysFile && pathExists homeOverlaysDir then
  +      if pathOverlays != "" && builtins.pathExists pathOverlays then overlays pathOverlays
  +      else if builtins.pathExists homeOverlaysFile && builtins.pathExists homeOverlaysDir then
           throw ''
             Nixpkgs overlays can be specified with ${homeOverlaysFile} or ${homeOverlaysDir}, but not both.
             Please remove one of them and try again.
           ''
  -      else if pathExists homeOverlaysFile then
  +      else if builtins.pathExists homeOverlaysFile then
           if isDir homeOverlaysFile then
             throw (homeOverlaysFile + " should be a file")
           else overlays homeOverlaysFile
  -      else if pathExists homeOverlaysDir then
  +      else if builtins.pathExists homeOverlaysDir then
           if !(isDir homeOverlaysDir) then
             throw (homeOverlaysDir + " should be a directory")
           else overlays homeOverlaysDir
  
positive.code: , # Fallback: The contents of the configuration file found at $NIXPKGS_CONFIG or
    # $HOME/.config/nixpkgs/config.nix.
    config ? let
        configFile = getEnv "NIXPKGS_CONFIG";
        configFile2 = homeDir + "/.config/nixpkgs/config.nix";
        configFile3 = homeDir + "/.nixpkgs/config.nix"; # obsolete
      in
        if configFile != "" && pathExists configFile then import configFile
        else if homeDir != "" && pathExists configFile2 then import configFile2
        else if homeDir != "" && pathExists configFile3 then import configFile3
        else {}
  
  , # Overlays are used to extend Nixpkgs collection with additional
    # collections of packages.  These collection of packages are part of the
    # fix-point made by Nixpkgs.
    overlays ? let
        isDir = path: pathExists (path + "/.");
        pathOverlays = try (toString <nixpkgs-overlays>) "";
        homeOverlaysFile = homeDir + "/.config/nixpkgs/overlays.nix";
        homeOverlaysDir = homeDir + "/.config/nixpkgs/overlays";
  
negative.code: /* Impure default args for `pkgs/top-level/default.nix`. See that file
     for the meaning of each argument. */
  
  
  let
  
    homeDir = builtins.getEnv "HOME";
  
    # Return ‘x’ if it evaluates, or ‘def’ if it throws an exception.
    try = x: def: let res = builtins.tryEval x; in if res.success then res.value else def;
  
  in
  
  