<xsl:sort select="$sort.key"/>

A variable holds a value not an expression so this would sort everything
with the same value (so not sort).

Just directly code it closer to your english description

for example 

  If a record has a 
  field[@id='130'] use it as the sort key if there is no field[@id='130'] 
  then use the field[@id='245']

 could be coded as

   if(field[@id='130']) then field[@id='130'] else field[@id='245']

which would work in xsl:sort, it could be coded in less keystrokes as

(field[@id='130'],field[@id='245'])[1]


but there's something to be said for making the code match the problem
spec, and leave the term re-writing to the xslt optimiser.

David


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