Hello Jean-Philippe, Wednesday, January 04, 2006, 6:36:14 PM, you wrote: JPB> We can have high-level types/API for maps and sets that rely on a JPB> sound Ord instance and in return guarantees validity of the structure; JPB> and a low level api for AVL trees that allows everything. imho, it the best way. for rather complex datastructures it is best to implement in low-level module just the all operations this datastructure provides and add high-level modules, which models some abstract data types (Map, Set, Foldable and so on) over this implementation. moreover, imvho it is better to leave low-level implementation open (i.e. export all functions and internal datastructure) so that anyone can add new low-level functions to this datatype (say, serialization), or use this low-level api to implement new high-level ADTs -- Best regards, Bulat mailto:bulatz@HotPOP.com