> I agree the above is O(n^2) with no duplicates. However I wonder how
distinct-values is implemented with Saxon that you say it is O(n log n). What
intrigues me is that the order does not change on Oxygen 16.1.
>

Saxon's implementation of distinct-values($input) is basically:

HashSet set = new HashSet();
for each x in $input
  if (!set.contains(x)) {
    set.add(x);
    result.add(x);
  }
}

except that the code is inverted to make it streamable.