{
  inputs,
  lib,
  system,
  ...
}:
with lib;
rec {
  my = import ./my.nix { inherit lib inputs system; };

  dns = import ./dns.nix { inherit lib inputs system; };

  packages = import ./packages.nix { inherit lib inputs system; };

  isEven =
    number:
    assert (builtins.isInt number) || (builtins.isFloat number);
    builtins.div number 2 == 0;

  isOdd = number: !isEven number;

  pow =
    base: exponent:
    assert (builtins.isInt base) && (builtins.isInt exponent);
    assert exponent > 0;
    builtins.foldl' (x: _: x * base) 1 (builtins.genList (_: _) exponent);

  mkTcpMem =
    min: ini: max:
    assert min <= ini && ini <= max;
    concatMapStrings (x: toString x + " ") (
      map (pow 2) [
        min
        ini
        max
      ]
    );

  # Load all files from directory (partially recursively).
  #
  # Usage: Assuming other Nix files are present in a directory:
  # ```
  # { ... }: {
  #   imports = attrValue (modulesIn ./.);
  #
  #   services.foobar.enable = true;
  # }
  # ```
  #
  modulesIn =
    dir:
    pipe dir [
      builtins.readDir
      (mapAttrsToList (
        name: type:
        let
          modulePath = dir + "/${name}";
        in
        if type == "regular" && hasSuffix ".nix" name && name != "default.nix" then
          [ (nameValuePair (removeSuffix ".nix" name) modulePath) ]
        else if type == "directory" && pathExists (modulePath + "/default.nix") then
          [ (nameValuePair name modulePath) ]
        else
          [ ]
      ))
      concatLists
      listToAttrs
    ];

  # Override a module using another Nixpkgs source tree.
  #
  # Usage: Assuming we want to override "services.foobar.enable":
  # ```
  # { config, lib, ... }:
  # (lib.modulesFromRef
  #   "services/security/foobar.nix"
  #   "azahi:fix-foobar" # Can be copied form a PR.
  #   "sha256-AAA..."
  # ) // {
  #   config = {
  #     serices.foobar.enable = true;
  #   };
  # }
  # ```
  # or
  # ```
  # nixosConfigurations.machine = nixosSystem {
  #   modules = [
  #     (_: moduleFromRef "services/security/foobar.nix" "azahi:foobar-fix" "sha256-AAA...")
  #   ];
  # };
  # ```
  #
  moduleFromRef = module: ref: sha256: {
    disabledModules = [ module ];
    imports = [
      (
        let
          src = inputs.nixpkgs.legacyPackages.${system}.fetchzip {
            url =
              let
                cons = splitString ":" ref;
                owner = head cons;
                branch = last cons;
              in
              "https://github.com/${owner}/nixpkgs/archive/refs/heads/${branch}.tar.gz";
            inherit sha256;
          };
        in
        "${src}/nixos/modules/${module}"
      )
    ];
  };

}