sorted for-each: preceding::sibling is not in sort ord

Working on a grouping problem I was trying to use preceding::sibling
on a selection of nodes that I had assembled with for-each and sorted.

It took me a long time to find out that while for-each will present the
nodes in sort order, preceding::sibling will give me the
results for a totally different node (probably document order).

Is this intended behaviour (I'm working with Xalan)? And if so, what is
the best way around this. The only way I found was to do a first
for-each copy-of all the nodes into a variable, then work with the
content of the variable in a second for-each using that variable.

One big disadvantage being that I'm cutting off all the parent
relationships when copying my node.

Thanks for your tips,

--
Ferdinand Soethe