anchor.diff:      proxy_set_header        Accept-Encoding "";
     '';
   
  +  upstreamConfig = toString (flip mapAttrsToList cfg.upstreams (name: upstream: ''
  +    upstream ${name} {
  +      ${toString (flip mapAttrsToList upstream.servers (name: server: ''
  +        server ${name} ${optionalString server.backup "backup"};
  +      ''))}
  +    }
  +  ''));
  +
     configFile = pkgs.writeText "nginx.conf" ''
       user ${cfg.user} ${cfg.group};
       error_log stderr;
  
positive.code:       ${optionalString (cfg.resolver.addresses != []) ''
          resolver ${toString cfg.resolver.addresses} ${optionalString (cfg.resolver.valid != "") "valid=${cfg.resolver.valid}"};
        ''}
  
        ${optionalString (cfg.recommendedOptimisation) ''
          # optimisation
  
negative.code:         default = {};
        };
  
        upstreams = mkOption {
          type = types.attrsOf (types.submodule {
            options = {
              servers = mkOption {
                type = types.attrsOf (types.submodule {
                  options = {
                    backup = mkOption {
                      type = types.bool;
                      default = false;
                      description = ''
                        Marks the server as a backup server. It will be passed
                        requests when the primary servers are unavailable.
                      '';
                    };
                  };
                });
                description = ''
                  Defines the address and other parameters of the upstream servers.
                '';
                default = {};
              };
            };
          });
          description = ''
            Defines a group of servers to use as proxy target.
          '';
          default = {};
        };
  
        virtualHosts = mkOption {
          type = types.attrsOf (types.submodule (import ./vhost-options.nix {
            inherit config lib;
  