value of attribute from defined variable

xml:

<alt john="the price is 2 dollars" jack="the price is 2.25
dollars"/>

xsl current:
  <xsl:template match="alt">
    <xsl:if test="$output='john'">
     <xsl:value-of select="@john"/>
    </xsl:if>
    <xsl:if test="$output='jack'">
     <xsl:value-of select="@jack"/>
    </xsl:if> 
   </xsl:template> 


xsl desired - or something to this effect:
  <xsl:template match="alt">
     <xsl:value-of select="@{$output}/>
 </xsl:template>

How do I identify the desired attribute by using the variable
$output (which is either 'john' pr 'jack'
-- 
Eric Smith - currently using xalan and fop on linux

 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list