>> >> +(defcustom Buffer-menu-group-by nil
>> >> +  "If non-nil, buffers are grouped by function."
>> >> +  :type 'function
>> >> +  :group 'Buffer-menu
>> >> +  :version "30.1")
>> >
>> > Please consider letting users choose a symbol, not a function.  Each
>> > symbol can be mapped to a function, but having a user option whose
>> > values are functions makes it harder for users to customize the
>> > option.
>>
>> Ok, now the type is changed to 'symbol'.  Here is the complete patch:
>
> Thanks, but I wonder if we can be a tad more user-friendly?  The user
> option exists, and it can accept simple enough data structures for
> customizations, but the default is nil, and there's no example or
> pre-cooked list of ready-to-use values anywhere in sight, not even in
> the doc string.  Users will have to work hard to produce a grouping of
> their liking.  E.g., suppose I want to group buffers by mode -- how
> would I go about it?  Or what about grouping buffers by their leading
> directories -- how can that be done?  Etc. etc. -- this could be a
> very powerful feature, but we should make its use easier.  And, after
> reading the documentation of tabulated-list-groups and that of
> tabulated-list-entries several times, I have no idea how to specify
> simple groupings such as those described above.  Which makes examples
> or pre-cooked values even more important, IMO.

Ok, then this patch adds two pre-cooked functions as a starting point
for users to understand the principle.  Then users will propose
more functions and we will choose the most useful candidates
for including to the set of default functions later.