anchor.diff:  , # Fallback: The contents of the configuration file found at $NIXPKGS_CONFIG or
     # $HOME/.config/nixpkgs/config.nix.
     config ? let
  -      configFile = getEnv "NIXPKGS_CONFIG";
  +      configFile = builtins.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
  +      if configFile != "" && builtins.pathExists configFile then import configFile
  +      else if homeDir != "" && builtins.pathExists configFile2 then import configFile2
  +      else if homeDir != "" && builtins.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 + "/.");
  +      isDir = path: builtins.pathExists (path + "/.");
         pathOverlays = try (toString <nixpkgs-overlays>) "";
         homeOverlaysFile = homeDir + "/.config/nixpkgs/overlays.nix";
         homeOverlaysDir = homeDir + "/.config/nixpkgs/overlays";
  
positive.code:         # 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
            map (n: import (path + ("/" + n)))
              (builtins.filter (n: builtins.match ".*\\.nix" n != null || pathExists (path + ("/" + n + "/default.nix")))
                (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
          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
          if isDir homeOverlaysFile then
            throw (homeOverlaysFile + " should be a file")
          else overlays homeOverlaysFile
        else if pathExists homeOverlaysDir then
          if !(isDir homeOverlaysDir) then
            throw (homeOverlaysDir + " should be a directory")
          else overlays homeOverlaysDir
  
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
  
  