If something I'll call lists are implemented following the model of categories (that is an element can belong to multiple, arbitrarily named entities) and rollup is extended to allow that as an option in addition to the parent element, this would allow many level rollup without the cost of recursion/tree-walking. A simple utility for list maintenance could be built that could do the recursion if that is viewed to be useful since performance would not be a problem for a one-off operation.
