acccessing and changing result tree nodes

How do I access result tree nodes and change the value
based upon some conditions?
Normally all match or test attributes point to source
tree.
 
I have a situation where in the same xsl file, an
attribute is being generated somewhere deep in the
result tree. This attribute is also being copied from
source tree to result in some other places in the
tree. After all this processing is complete, I now
must recheck the value of this attribute in the result
tree (no matter where in the tree it occurs) and  if
it is emtpy, make it equal to the value of another
attribute that is also present in the same element and
may have itself be generated anew or copied from
source to result tree.
 
I know one way is to split the process into 2 stages
and create a second xsl file but I am looking for
above solution for performance reasones.
 
Thanks,
Alan

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