It'd be possible to get this behavior with XSL.
The Description is normally not displayed in XML results, so you'll need an Additional Field that's populated with the Description (and set to display), and Abstract Style set to Query.
Then you'll have both Abstract and Description present, and you can use a <xsl:choose> structure similar to the previous ones to test if Description is empty.
The Description is normally not displayed in XML results, so you'll need an Additional Field that's populated with the Description (and set to display), and Abstract Style set to Query.
Then you'll have both Abstract and Description present, and you can use a <xsl:choose> structure similar to the previous ones to test if Description is empty.