anchor.diff:  
   let 
     inherit (pkgs.lib) mkOption mkIf;
  +
     cfg = config.services.gvpe;
  +
     finalConfig = if cfg.configFile != null then 
       cfg.configFile
     else if cfg.configText != null then
  
positive.code:     }
    else 
      throw "You must either specify contents of the config file or the config file itself for GVPE";
    ifupScript = if cfg.ipAddress == null || cfg.subnet == null then 
       throw "Specify IP address and subnet (with mask) for GVPE" 
     else if cfg.nodename == null then 
  
negative.code:         chown root /var/gvpe
          chmod 700 /var/gvpe
          cp ${finalConfig} /var/gvpe/gvpe.conf
          cp ${ifupScript} /var/gvpe/if-up
        '';
  
        inherit exec;
  