{ pkgs }: {
  packageOverrides = super: let
    self = super.pkgs;
    pkgsUnstable = if super.pkgs.stdenv.isLinux
      then import <nixos-unstable> {}
      else super.pkgs;

  in with self; rec {
    myEmacs = super.callPackage ./emacs.nix {
      emacs = if stdenv.isDarwin then emacs25Macport else emacs;
    };
    nodejs = pkgsUnstable.nodejs-8_x;

    zr = super.callPackage ./zr.nix {};
  };

  allowUnfree = true;
}