Conditionally processing optional elements

 In creating an XSL, I have this problem: I have an
 XML file that may or may not contain a certain
 element. If it exists then a child element will
 reference it, otherwise the child must refernce
 another element. I'm not sure how to specify
 
 If optional element exists then Child element
 references optional element  Else child element
 references other required element
 


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