> I currently decide where to get it from using an xsl:choose
> each time I need data, however,  it would be much more
> efficient if I could save the correct node-set into a variable.

Just to explain why David Carlisle's solution works and yours doesn't, if
you use xsl:choose then you are creating a tree that contains copies of data
from the source node, you are not creating a node-set containing references
to the original nodes. So the general way of defining a node-set
conditionally is

<xsl:variable name="ns" select="$a[$condition] | $b[not($condition)]"/>

Mike Kay


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