bug#41706: 26.1; sort-subr predicate cannot be set successfully

[Eli Zaretskii <eliz@gnu.org>]

> From: Michael Heerdegen <michael_heerdegen@web.de>
> Cc: post+ebugs@guelker.eu,  41706@debbugs.gnu.org
> Date: Wed, 10 Jun 2020 20:59:30 +0200
> 
> +STARTKEYFUN moves from the start of the record to the start of
> +the key.  It may return either a non-nil value to be used as the
> +key, or else the key is a cons (BEG . END) designating the
> +substring of the record between the values of point after
> +STARTKEYFUN and ENDKEYFUN are called.  If STARTKEYFUN is nil, the
> +key starts at the beginning of the record.

That's a beginning, thanks.  But it needs more work, IMO:

 . the cons cell case hints on a possible return value of STARTKEYFUN,
   AFAIU, but the text doesn't say so
 . the case of STARTKEYFUN being nil is another use case for what
   PREDICATE must cope with, and the text should say so
 . the text about ENDKEYFUN should also be augmented, IMO
 . the description of PREDICATE should reference this text in some
   useful way

> BTW, what about the suggestion to support collation order out of the
> box?

What collation would you like to support, and in what form?