[xsl] display a text if element does not exist

["Paul S" <b0nn19@xxxxxxxxxxx>]

Hi,

I have a XML like this

<students>
  <student>
     <name>John</name>
     <ID>1342</ID>
     <course>MATH</course>
  </student>

 <student>
     <name>Marry</name>
     <ID>1526</ID>
  </student>
</students>

And I want to display a text if course tag does not exists. How can I do 
this?

Thanks.

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