By writing a <SubConcept> node in this form, the output xml file ensures 
  that a <SubConcept> node definitely exists. However, I want the output 
 xml file to have a <SubConcept> node only if

well stick an xsl:if around it that does that test then.

  One way I was thinking of coming round 
  this problem is to declare a global variable and then assign it a value 
  locally 

XSLT doesn't have separate declaration and binding of variables.
If you have declared a variable it already has a value (and it you can't
give it another one.)


David



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