py-billow
several python types: nil, ring, pair, treedict
Module have several types in python: ring (double linked list), pair (light weight tuple), Nil-singletone, treedict (avl & rb balanced binary trees).
In billow SQL(NULL) for now is 'billow.Nil' type with some aggregate functions on cpython release, and 'expectone' as bridge aggregated function together with 'coalesce', 'count', 'min', 'max', 'minmax'.