anchor.diff:    options = {
   
       services.xserver.displayManager.kdm = {
  +    
         enable = mkOption {
           default = false;
           description = ''
             Whether to enable the KDE display manager.
           '';
         };
  +
  +      extraConfig = mkOption {
  +        default = "";
  +        description = ''
  +          Options appended to <filename>kdmrc</filename>, the
  +          configuration file of KDM.
  +        '';
  +      };
  +      
       };
   
     };
  
positive.code: 
    inherit (pkgs.kde42) kdebase_workspace;
  
    kdmrc = pkgs.stdenv.mkDerivation {
      name = "kdmrc";
      # -e "s|Session=${kdebase_workspace}/share/config/kdm/Xsession|Session=${dmcfg.session.script}|" \
      buildCommand = ''
        cp ${kdebase_workspace}/share/config/kdm/kdmrc .
        sed -i -e "s|#HaltCmd=|HaltCmd=${pkgs.upstart}/sbin/halt|" \
               -e "s|#RebootCmd=|RebootCmd=${pkgs.upstart}/sbin/reboot|" \
  	     -e "s|#Xrdb=|Xrdb=${pkgs.xlibs.xrdb}/bin/xrdb|" \
  	     -e "s|#HiddenUsers=root|HiddenUsers=root,nixbld1,nixbld2,nixbld3,nixbld4,nixbld5,nixbld6,nixbld7,nixbld8,nixbld9,nixbld10|" \
  	     -e "s|ServerCmd=/FOO/bin/X|ServerCmd=${dmcfg.xserverBin} ${dmcfg.xserverArgs}|" \
   	     -e "s|^SessionsDirs=.*$|SessionsDirs=${dmcfg.session.desktops}|" \
  	     -e "s|#FailsafeClient=|FailsafeClient=${pkgs.xterm}/bin/xterm|" \
  	     -e "s|#PluginsLogin=sign|PluginsLogin=${kdebase_workspace}/lib/kde4/kgreet_classic.so|" \
        kdmrc
        ensureDir $out
        cp kdmrc $out
      '';
    };
  
  in
  
negative.code:     services.xserver.displayManager.job =
        { beforeScript = "";
          env = "";
          execCmd = "${kdebase_workspace}/bin/kdm -config ${kdmrc}";
        };
  
      security.pam.services = [ { name = "kde"; localLogin = true; } ];
  