I will try to post to this mailing list, thanks for the link. Indeed I started to implement through xquery a map version able to handle nodes for my needs. It is not obvious, because a map requires to define an equality properties over set. For nodes, deep-equal is a natural candidate, but it does not define an ordering, and performances are lost. For database nodes, it is possible to achieve this with internal (or maybe pre-database) ids, but I can't use it for dynamically created nodes. I guess that the best choice is to provide an optional binding to an external ordering function, something like map:new($maps as map(*)*, binding as map(*)) as map(*), where the binding provides an option to the ordering function (e.g. $order(key1,key2) := binding["order"](key1,key2)). This is actually what is done for C++ or Java sets : 

template < class T,                        // set::key_type/value_type
           class Compare = less<T>,        // set::key_compare/value_compare
           class Alloc = allocator<T> >    // set::allocator_type
           > class set;

>> - A remark of minor importance : if distinct-values() is used to select