Try
<xsl:template match="Array">
<xsl:variable name="retain" select="ArrayElement[not(. = 'BBBBBB')]"/>
<xsl:attribute name="length">
<xsl:value-of select="count($retain"/>
</xsl:attribute>
<xsl:copy-of select="$retain"/>
</xsl:template>
Michael Kay
http://www.saxonica.com/
Author, XSLT Programmer's Reference and XPath 2.0 Programmer's Reference