File: Data.xml
A
B
C
D
E
F
G
H
I
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L
File: Transform.xslt
xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
group-by="position() mod (last() idiv $cols)">
Output:
ml version="1.0" encoding="UTF-8"?>
A
D
G
J
B
E
H
K
C
F
I
L