The type system of xslt 2.0 is not complete (see Sequence of sequences in xslt 2.0). You cannot perform manipulations over items as you could do. The reason is in the luck of set based constructs: xslt 2.0 supports sequences, but not associative maps of items.

If you think that xml can be used as a good approximation of a map, I shan't agree with you. Xml has an application in a very specific cases only. Maps I'm thinking of,  would allow associate items by reference, like sequences do.

This opens a perspective to create a state objects, to manage sequence of sequences, to create cyclic graphs of items, and so on. These maps are richer than what key() function provides right now, and allow to implement for-each-group in xquery.

Such maps can be modeled with several functions, however I would wish they were built in:

f:map($items as item()*) as item()
Returns a map from a sequence $items of pairs (key, value).

f:map-items($map as item()) as item()*
Returns a sequence of pairs (key, value) for a map $map.

f:map-keys($map as item()) as item()*
Returns a sequence of keys contained in a map $map.

f:map-values($map as item()) as item()*
Returns a sequence of values contained in a map $map.

f:map-value($map as item(), $key as item()) as item()*
Returns a sequence of values corresponding to a specified key $key contained a specified map $map.

The other thing I would add is items tuple. It's like a sequence, however a sequence of tuples is never transformed into single sequence, but stays as sequence of tuples.

Fortunately it's possible to implement such extension functions.