anchor.diff:  
       system.build.manual = manual;
   
  -    environment.systemPackages = [ manual.manpages ];
  +    environment.systemPackages = [ manual.manpages help ];
   
       boot.extraTTYs = mkIf cfg.showManual ["tty${cfg.ttyNumber}"];
   
  
positive.code:         { description = "NixOS Manual";
            wantedBy = [ "multi-user.target" ];
            serviceConfig =
              { ExecStart = "${cfg.browser} ${manual.manual}/share/doc/nixos/manual.html";
                StandardInput = "tty";
                StandardOutput = "tty";
                TTYPath = "/dev/tty${cfg.ttyNumber}";
  
negative.code:     inherit (cfg) revision;
      inherit pkgs options;
    };
  
    entry = "${manual.manual}/share/doc/nixos/manual.html";
  
    help = pkgs.writeScriptBin "nixos-help"
      ''
        #! ${pkgs.stdenv.shell} -e
        if ! ''${BROWSER:-w3m} ${entry}; then
          echo "$0: unable to start a web browser; please set \$BROWSER or install ‘w3m’"
          exit 1
        fi
      '';
  
  in
  
  