That's a very good point. So we don't have a set of sets -- we have a set of lists (where normal constructors -- which have no overlap -- would appear in the lists in every possible permutation).

Again, please don't take my set of lists too seriously from an implementation point of view. We clearly wouldn't implement it this way. But I want to use this abstraction to understand the shape of the problem and what an idealized solution might look like before worrying about implementation and syntax.

Richard