Notice how compare/2 passes undefined to compare/3 as the AccFun. That would make it use hashtree:compare/2, which defaults to an accumulator function that concatenates lists. Also notice how compare/2 is not used anywhere in the Riak tree. If something were to happen to compare/2, I would not tell anybody.  If someone was even more caremad and trimmed all these different "convenient" versions of compare through these modules, I wouldn't say anything either.