pg_xlog - files are guaranteed to be sequentialy named?

Hi pgsql-list-members,
I currently write a small script that deletes outdated xlog-files from
my backup-location.
Because I do not want to rely on creation-date, I found it usable to use
the result of
ln | sort -g -r
Thus the newest WAL xlog-file is on top and I can delete all not needed
files at the bottom of the list.

My question: Is it for ALL cases guaranteed, that the naming of the
WAL-files in $PGDATA/pg_xlog always produces a "higher" number for a
newer file?
What happens if the 24hexdigits reach upper bound?

Thank your for your replies on that issue of postgresql inner working model.
Regards Johannes