Document function -- document may not exist

I was wondering if there is a way of getting around the exception that is
thrown by most parsers if the document function is called for a file that
does not exist.  I would just like to continue on in the processing.  For
example, I would like to do something like the following:

<xsl:variable name="FILE_A" select="document('A.xml')"/>
<xsl:variable name="FILE_B" select="document('B.xml')"/>

<xsl:for-each select="$FILE_A">
     <!-- If it exists, do some processing -->
     ...
</xsl:for-each>
<xsl:for-each select="$FILE_B">
     <!-- Same.... -->
</xsl:for-each>

Thanks in advance,
Matt


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