anchor.diff:  /* Impure default args for `pkgs/top-level/default.nix`. See that file
      for the meaning of each argument. */
   
  -with builtins;
   
   let
   
     homeDir = builtins.getEnv "HOME";
   
     # Return ‘x’ if it evaluates, or ‘def’ if it throws an exception.
  -  try = x: def: let res = tryEval x; in if res.success then res.value else def;
  +  try = x: def: let res = builtins.tryEval x; in if res.success then res.value else def;
   
   in
   
  
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: , # Fallback: The contents of the configuration file found at $NIXPKGS_CONFIG or
    # $HOME/.config/nixpkgs/config.nix.
    config ? let
        configFile = builtins.getEnv "NIXPKGS_CONFIG";
        configFile2 = homeDir + "/.config/nixpkgs/config.nix";
        configFile3 = homeDir + "/.nixpkgs/config.nix"; # obsolete
      in
        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: builtins.pathExists (path + "/.");
        pathOverlays = try (toString <nixpkgs-overlays>) "";
        homeOverlaysFile = homeDir + "/.config/nixpkgs/overlays.nix";
        homeOverlaysDir = homeDir + "/.config/nixpkgs/overlays";
  