{ pkgs ? import <nixos-unstable> {} } :
{

  pkgs = import (builtins.fetchGit {
      name = "nixos-23.05";
    url = "https://github.com/nixos/nixpkgs/";
    ref = "refs/heads/nixos-unstable";
    rev = "ef99fa5c5ed624460217c31ac4271cfb5cb2502c";
  });

  foo = pkgs.mkShell {
    # nativeBuildInputs is usually what you want -- tools you need to run
    nativeBuildInputs = with pkgs.buildPackages; [ 

      mmark
      xml2rfc
      python312Packages.lxml
      wkhtmltopdf-bin
      imagemagick

    ];
  };

  # to create [markdown] table of contents use LLM with this input: awk '/id="/ { print $0 }' RFC_XR_Fragments.html | grep -v idx
}