anchor.diff:  
   let
     cfg = config.services.amule;
  +  user = if cfg.user != null then cfg.user else "amule";
   in
   
   {
  
positive.code: 
          preStart = ''
              mkdir -p ${cfg.dataDir}
              chown ${cfg.user} ${cfg.dataDir}
          '';
  
          exec = ''
              ${pkgs.su}/bin/su -s ${pkgs.stdenv.shell} ${cfg.user} \
                  -c 'HOME="${cfg.dataDir}" ${pkgs.amuleDaemon}/bin/amuled'
          '';
        };
  
      environment.systemPackages = [ pkgs.amuleDaemon ];
  
    };
    
  }
  
negative.code: 
    config = mkIf cfg.enable {
  
      users.extraUsers = mkIf (cfg.user == null) [
        { name = cfg.user;
          description = "AMule daemon";
        } ];
  
      jobs.amuled =
        { description = "AMule daemon";
  