{
  lib,
  pkgs,
  config,
  ...
}:
with lib; let
  # WORK IN PROGRESS
  # WORK IN PROGRESS
  # WORK IN PROGRESS
  # Shorter name to access final settings a
  # user of hello.nix module HAS ACTUALLY SET.
  # cfg is a typical convention.
  cfg = config.services.hello;
in {
  # Declare what settings a user of this "hello.nix" module CAN SET.
  options.services.hello = {
    enable = mkEnableOption "hello service";
    greeter = mkOption {
      type = types.str;
      default = "world";
    };
  };

  # Define what other settings, services and resources should be active IF
  # a user of this "hello.nix" module ENABLED this module
  # by setting "services.hello.enable = true;".
  config = mkIf cfg.enable {
    systemd.services.hello = {
      wantedBy = ["multi-user.target"];
      serviceConfig.ExecStart = "${pkgs.hello}/bin/hello -g'Hello, ${escapeShellArg cfg.greeter}!'";
    };
  };
}