I have a field called "Audio File" it is a Download Type.   I use xslt script to reformat the list, but i need to add an if/then or choose/when/otherwise. 
psudo code---
if the "Audio File"  URL is blank (nothing downloaded).. then dont show the link and its text
else show audio file linktext
End psudo code---
 
Here is what i tried.  But i get a manditory test error

_________________Solved_______________---
Ok i am guilty of... "I don't want to learn, i just want someone to tell me the answer"
It didnt take that much to learn the answer.  Here it is, if someone is searching for this.

<xsl:variable name="AudioURLExists" select="udt:Audio_UDT_Url" />
          <xsl:choose>
            <xsl:when test="$AudioURLExists=''">
            </xsl:when>
            <xsl:otherwise>
              <a href="{udt:Audio_UDT_Url}">
                <img src="http://mydomain.org/portals/1326/icon..." border="0" />
              </a>
            </xsl:otherwise>
          </xsl:choose>