anchor.diff:    emacs-overlay = import (builtins.fetchGit {
       url = "git@github.com:nix-community/emacs-overlay.git";
       ref = "master";
  -    rev = "7448efeddfa7ac44dceffbb43f70cb5d5ecb7385"; # pinned to 02/12/2020
  +    rev = "020d3009feffb10bb3d43d3b69882ee7fc9a2b50"; # pinned to 02/27/2020
     });
   
     pkgs = pkgs-darwin;
     hub = pkgs.gitAndTools.hub;
     pinentry = if (pkgs.stdenv.isDarwin) then pkgs.pinentry_mac else pkgs.pinentry;
  +
  +  # Bleeding edge Emacs slows way down in fullscreen on MacOS; running the
  +  # latest Emacs 27 seems to work okay.
  +  emacs27 = pkgs.emacsGit.overrideAttrs(old: {
  +    src = pkgs.fetchFromGitHub {
  +      owner = "emacs-mirror";
  +      repo = "emacs";
  +      rev = "f9e53947c71ddf7f8d176a23c0a585fafa13b952"; # emacs-27 branch
  +      sha256 = "1hjnvbpa7iirh2hgqw707az13788hjffkg29r74hk9pikmyq0dxz";
  +    };
  +  });
  +
  +  # See: https://nixos.org/nixos/manual/index.html#module-services-emacs-adding-packages
  +  myEmacsWithPackages = (pkgs.emacsPackagesGen emacs27).emacsWithPackages;
  +
  +  # Currently vterm support requires building with the 'libvterm' library and
  +  # the 'emacs-libvterm' package. Longer term, all packages should be moved here
  +  # and pinned to the appropriate version of 'emacs-overlay'
  +  emacs = myEmacsWithPackages (epkgs: (with epkgs.melpaPackages; [
  +    vterm
  +  ]));
   
     custom = rec {
       inherit (pkgs) callPackage;
  
positive.code:   allPlatforms = with custom;
      [
        # Customized packages
        fzf
        git
        hub
  
negative.code: 
    # The list of packages to be installed
    tools = allPlatforms;
  in
    if pkgs.lib.inNixShell
    then pkgs.mkShell
  