Why does this XQuery (Mergesort) Code Snippet not work correct?

[Brob]

While learning XQuery I'm trying to implement a merge sort. For joining two already sorted sequences into one I wanted to implement a helper function called "merge". It currently looks like this: \[code\]declare function local:merge($seq1 as xs:integer*, $seq2 as xs:integer*) as xs:integer*{if(empty($seq1) and empty($seq2)) then ()else if(empty($seq1)) then $seq2else if(empty($seq2)) then $seq1elselet $zahl1 := $seq1[1]let $zahl2 := $seq2[2]return if($zahl1 < $zahl2) then ($zahl1, local:merge(fn:subsequence($seq1,2), $seq2))else if($zahl2 < $zahl1) then ($zahl2, local:merge($seq1, fn:subsequence($seq2,2)))else ($zahl1, $zahl2, local:merge(fn:subsequence($seq1,2), fn:subsequence($seq2,2)))};\[/code\]I'm testing my code with this call:\[code\]local:merge((1,3,6,9,10), (1,2,4,6,8,11))\[/code\]Somehow the result isn't always correct (under this is the result for the call from above):\[code\] 1 2 3 4 6 6 8 9 10 8 11\[/code\]The idea behind this was that I always check if the first number of a sequence is bigger or smaller than the one in the other sequence. I would then add the smaller number to my result, cut the sequence with the smaller number into a subsequence without the just added number and go again recursively. The break condition are when either one of the two sequences is empty or both. I can't see my mistake at the moment, perhaps it's something little that I'm overlooking. Can you show me where I need to change my code that it works correct? Thanks a lot for your help.