I had to convert the file to lexical binding to avoid a quoted lambda,
or else we had been forbidden to name the optional argument "predicate"
(variable name clash).

> My opinion is the opposite: I think it's more important to have a
> command that could collate-order strings according to a user-specified
> locale, than make sort-lines more flexible on the Lisp level.

What would you do?  Just create an additional command?


Michael.