anchor.diff:  
   let
     cfg = config.services.amule;
  +  user = if cfg.user != null then cfg.user else "amule";
   in
   
   {
  
positive.code:       };
  
        dataDir = mkOption {
          default = ''/home/${cfg.user}/'';
          description = ''
            The directory holding configuration, incoming and temporary files.
          '';
        };
  
        user = mkOption {
          default = "amule";
          description = ''
            The user the AMule daemon should run as.
          '';
  
negative.code: 
    config = mkIf cfg.enable {
  
      users.extraUsers = mkIf (cfg.user == null) [
        { name = cfg.user;
          description = "AMule daemon";
        } ];
  
      jobs.amuled =
        { description = "AMule daemon";
  