-o When merging, choose the output file with less runs instead of always switching?
-o Implement multi-way merge.
-o Mode with only 2-way unification?
-o Speed up 2-way merge.
-o Speed up radix splitting.
-o A debug switch for disabling the presorter.
+o Deal with too rough range estimates in radix splitting.
+o How does the speed of radix splitting decrease with increasing number of hash bits?
+ Does it help to use more bits than we need, so that we sort less data in memory?
+o Log messages should show both original and new size of the data. The speed
+ should be probably calculated from the former.