anchor.diff:  
   let
     configDir = "/var/lib/foobar";
  +  mqttUsername = "homeassistant";
     mqttPassword = "secret";
   in {
     name = "home-assistant";
  
positive.code:           elevation = 0;
          };
          frontend = {};
          # uses embedded mqtt broker
          mqtt.password = mqttPassword;
          binary_sensor = [{
            platform = "mqtt";
            state_topic = "home-assistant/test";
  
negative.code: 
    nodes.hass = { pkgs, ... }: {
      environment.systemPackages = with pkgs; [ mosquitto ];
      services.mosquitto = {
        enable = true;
        users = {
          "${mqttUsername}" = {
            acl = [ "pattern readwrite #" ];
            password = mqttPassword;
          };
        };
      };
      services.home-assistant = {
        inherit configDir;
        enable = true;
  