{pkgs}: {
  isGitLfsSetup = checkFile: let
    gitLfsCheckOutput = builtins.readFile (pkgs.runCommand "gitLfsCheck" {} ''
      result="$(${pkgs.file}/bin/file --mime-type ${checkFile})"
      (if [ "$result" = "${checkFile}: image/jpeg" ]; then printf "0"; else printf "1"; fi) > $out
    '');
  in
    if gitLfsCheckOutput == "0"
    then true
    else
      throw ''
        Git Large File Storage (`git-lfs`) has not been set up correctly.
        Most common reasons:
          - `git-lfs` was not installed before cloning this repository.
          - This repository was not cloned from the primary GitLab mirror.
          - The GitHub mirror does not support LFS.
        See the book at https://book.veloren.net/ for details.
        Run `nix-shell -p git git-lfs --run "git lfs install --local && git lfs fetch && git lfs checkout"`
        or `nix shell nixpkgs#git-lfs nixpkgs#git -c sh -c "git lfs install --local && git lfs fetch && git lfs checkout"`.
      '';

  # Format number of seconds in the Unix epoch as %Y-%m-%d-%H:%M.
  dateTimeFormat = t: let
    rem = x: y: x - x / y * y;
    days = t / 86400;
    secondsInDay = rem t 86400;
    hours = secondsInDay / 3600;
    minutes = (rem secondsInDay 3600) / 60;
    seconds = rem t 60;

    # Courtesy of https://stackoverflow.com/a/32158604.
    z = days + 719468;
    era =
      (
        if z >= 0
        then z
        else z - 146096
      )
      / 146097;
    doe = z - era * 146097;
    yoe = (doe - doe / 1460 + doe / 36524 - doe / 146096) / 365;
    y = yoe + era * 400;
    doy = doe - (365 * yoe + yoe / 4 - yoe / 100);
    mp = (5 * doy + 2) / 153;
    d = doy - (153 * mp + 2) / 5 + 1;
    m =
      mp
      + (
        if mp < 10
        then 3
        else -9
      );
    y' =
      y
      + (
        if m <= 2
        then 1
        else 0
      );

    pad = s:
      if builtins.stringLength s < 2
      then "0" + s
      else s;
  in "${toString y'}-${pad (toString m)}-${pad (toString d)}-${pad (toString hours)}:${pad (toString minutes)}";
}