If statement and empty attribute

Hi,
I have an if statement which should evaluate to true when the attribute
viewable='true' or does not exist.  It would evaluate to false only when the
attribute viewable equals 'false'.  I've got the following:

        <xsl:if test="@viewable!='false'">
             <xsl:value-of select="$currentVAL"/>&nbsp;
        </xsl:if>

Which works, if the attribute viewable exists, but if it doesn't exist then
the statement is always false.  Any suggestions?  It needs to evaluate to
true if the attribute does not exist.

Karl


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