let
  fileModule = {
    config,
    name,
    gensokyo-zone,
    lib,
    ...
  }: let
    inherit (lib.options) mkOption mkEnableOption;
    inherit (lib.modules) mkOptionDefault;
    inherit (lib.strings) hasPrefix removePrefix;
  in {
    options = with lib.types; {
      enable =
        mkEnableOption "external file"
        // {
          default = true;
        };
      path = mkOption {
        type = str;
        default = name;
      };
      owner = mkOption {
        type = str;
        default = "root";
      };
      group = mkOption {
        type = str;
        default = "root";
      };
      mode = mkOption {
        type = str;
        default = "0644";
      };
      source = mkOption {
        type = path;
      };
      relativeSource = mkOption {
        type = nullOr str;
      };
    };
    config = {
      relativeSource = let
        flakeRoot = toString gensokyo-zone.self + "/";
        sourcePath = toString config.source;
      in
        mkOptionDefault (
          if hasPrefix flakeRoot sourcePath
          then removePrefix flakeRoot sourcePath
          else null
        );
    };
  };
in
  {
    config,
    gensokyo-zone,
    lib,
    ...
  }: let
    inherit (lib.options) mkOption;
  in {
    options.extern = with lib.types; {
      files = mkOption {
        type = attrsOf (submoduleWith {
          modules = [fileModule];
          specialArgs = {
            inherit gensokyo-zone;
            system = config;
          };
        });
        default = {};
      };
    };
  }