What I also would like to add to the docstring of this function, and of
that of `sort', is that the PREDICATE must be transitive and
antisymmetric - mentioning only in the manual is not enough IMHO.


> > BTW, what about the suggestion to support collation order out of the
> > box?
>
> What collation would you like to support, and in what form?

I don't know much about this stuff.  The canonical way from my ignorant
point of view would be that `compare-buffer-substrings' would not only
respect `case-fold-search' but also some other variable which would tell
how the behavior should be w.r.t. collation.

Michael.