This is not useless. Why would you have a list of lists? A list is just an iterable container class, which is what a set is. If the need exists for a list of l

This is not useless. Why would you have a list of lists? A list is just an iterable container class, which is what a set is. If the need exists for a list of lists then it exists for a set of sets. An example of this is finding all combinations possible non ordered groups of letters given a collection of letters: abadfjiik. A typical algorithm will produce dupes. But if your accumulator is a set of frozen sets you can produce a result with no dupes.