Discovered 28 February 2021:
Modern-day Linux sort takes the locale into account. But 4waylist.c does not. This may cause wrong results, because usually, the locale causes uppercase and lowercase file and directory names to be sorted together.

Solution: set and export LC_ALL=C before sorting the pathnames. Or it is probably fixable by making 4waylist.c locale aware too. Done it before somewhere, isn't hard to do. But I didn't do it yet.

See also: Four-way list.