xsl:when evaluated not=""

Part of my XSL looks like this:

        <xsl:choose>
          <xsl:when *** stuck here ***>
            <A>
              <xsl:attribute
name="href"><xsl:eval>getDirectory(this.getAttribute('page'))</xsl:eval></xs
l:attribute>
              <xsl:attribute name="target">new_window</xsl:attribute>
              <xsl:value-of select="@text"/>
            </A>
          </xsl:when>
          <xsl:otherwise>
            <xsl:value-of select="@text"/>
          </xsl:otherwise>
        </xsl:choose>

I only want the "when" to happen if
<xsl:eval>getDirectory(this.getAttribute('page'))</xsl:eval> returns some
value (that is, if it returns a blank value, I want to do the "otherwise").
I don't know the syntax for this.  Thanks.  Eric


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