* Optional Back End Functions:: Functions that need not be implemented.
* Error Messaging:: How to get messages and report errors.
* Writing New Back Ends:: Extending old back ends.
-* Hooking New Back ends Into Gnus:: What has to be done on the Gnus end.
-* Mail-like Back ends:: Some tips on mail back ends.
+* Hooking New Back Ends Into Gnus:: What has to be done on the Gnus end.
+* Mail-like Back Ends:: Some tips on mail back ends.
Various File Formats