Tree-fragments are heavily used when multiple passes are needed to compute a
result with only one stylesheet. However, you cannot use the key() function on
tree-fragments because XSLT doesn't allow you to (there is no nodeset
parameter)
To use result tree fragments at all as a node set you need to use an
xx:node-set() extension (as you have done) all implementations of
node-set that I have seen then return a tree rooted at a document
node. Keys may be used with such "temporary documents" just as with any
other node. key() will index in the document which contains the current
node so so long as the current node is a descendent of the root returned
by xx:node-set($x) the key will work as required.
David
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