Another point to consider is that if your a-list is very small (<= 5 elements) it could be faster than hash tables. The sharing behavior could be achieved with some sort of concatenated hash-tables, a list of tables to consider in turn to find the desired element. This seems very slow though. BTW, removing a-lists would be useless: they're so simple to implement that a lot of developers (me included) would re-invent them to use in their applications.
