This appears to be true as well.
I must have thought initially that nodes are immutable or something like that, which makes not too much sense - the update process wouldn't work the way I did it here then.

Guess the correct answer is that every XQuery node has a [unique node identity](http://www.w3.org/TR/xpath-datamodel/#node-identity) with full references to its place in a tree. Creating a copy in this line wouldn't help either, a deep-copy would be needed.