Hi Pascal,
it comes down to a syntax error - there's a 'return' statement missing before the 'copy'. It should work like this:
*declare namespace ddi = "http://www.icpsr.umich.edu/DDI%22;* *let $codeBook := collection()[1]/ddi:codeBook* *let $titlStmt := $codeBook/ddi:stdyDscr/ddi:citation/ddi:titlStmt* *return copy $titlStmt1 := $titlStmt* *modify (* * rename node $titlStmt1 as* *QName('http://www.icpsr.umich.edu/DDI%27,%27CopyOfTitlStmt%27)* *)* *return $titlStmt1* * * I always check queries in the Query Editor of BaseX if I come across issues like this. Just as a hint ...
Cheers, Lukas