anchor.diff:  let
     cfg = config.services.nginx;
     nginx = cfg.package;
  -  configFile = pkgs.writeText "nginx.conf" ''
  +  configFileText = ''
       user ${cfg.user} ${cfg.group};
       daemon off;
   
  
positive.code:   };
  
    config = mkIf cfg.enable {
      # TODO: test user supplied config file pases syntax test
  
      systemd.services.nginx = {
        description = "Nginx Web Server";
        after = [ "network.target" ];
  
negative.code:     ''}
      ${cfg.appendConfig}
    '';
    configFile = pkgs.runCommand "nginx.conf" {
      text = configFileText;
      passAsFile = ["text"];
      preferLocalBuild = true;
      allowSubstitutes = false;
    } ''
      mkdir -p "$(dirname "$out")"
      mv "$textPath" "$out"
      (${nginx}/bin/nginx -t -c "$out" -p ${cfg.stateDir} || true) 2>&1 | grep -q 'syntax is ok'
    '';
  
  in
  
  {
  