anchor.diff:  let
   
     inherit (pkgs.lib) mkOption;
  -  inherit (config) urlPrefix distDir;
   
  -  dataDir = "/data/pt-wiki/data";
  -  pubDir = "/data/pt-wiki/pub";
  +  dataDir = config.dataDir; # "/data/pt-wiki/data";
  +  pubDir = config.pubDir; # "/data/pt-wiki/pub";
   
     scriptUrlPath = "/bin";
     pubUrlPath    = "/pub";
  
positive.code:   dispScriptUrlPath = "";
    dispViewPath      = "";
  
    twikiName = "Program Transformation Wiki";
    startWeb = config.startWeb;
    registrationDomain = "ewi.tudelft.nl";
    defaultUrlHost = "";
  
    # Build the TWiki CGI and configuration files.
    twikiRoot = (import /etc/nixos/services/twiki/twiki-instance.nix).twiki {
      name = "pt-wiki";
      pubdir = pubDir;
      datadir = dataDir;
      inherit twikiName scriptUrlPath pubUrlPath absHostPath
        dispPubUrlPath dispScriptUrlPath dispViewPath
        registrationDomain defaultUrlHost;
    };
  
    plugins = import /etc/nixos/services/twiki/server-pkgs/twiki-plugins.nix;
  
negative.code:     Disallow: /pt/bin/
    '';
  
    options = {
  
      dataDir = mkOption {
        example = "/data/wiki/data";
        description = "
          Path to the directory that holds the Wiki data.
        ";
      };
  
      pubDir = mkOption {
        example = "/data/wiki/pub";
        description = "
          Path to the directory that holds uploaded files.
        ";
      };
  
      twikiName = mkOption {
        default = "Wiki";
        example = "Foobar Wiki";
        description = "
          Name of this Wiki.
        ";
      };
  
      registrationDomain = mkOption {
        example = "example.org";
        description = "
          Domain from which registrations are permitted.  Use `all' to
          permit registrations from anywhere.
        ";
      };
  
    };  
  
  }