IF test

Hello,
I want to check whether a arrtibute is present in a
element.For ex,
If there is a Book element, i want to check whether he
has got a ISBN attribute. ISBN is not a element.It
comes as a part of Book element.If a ISBN attribute is
not found then i'm planning to give it a new Code
attribute.
I have problems in finding whether it is present or
not.I used 
<xsl:template match="Book">
    <xsl:if test="not(@ISBN)">
       .
       .
       .
</xsl:template>

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