<xsl:apply-templates select="B">
  <xsl:sort select="B" order="ascending"/>

B doesn't have B children so the sort key is the empty set and so no
sort will occur.


<xsl:template match="B">
  <xsl:if test="not(position()=last())">


Because there is no sorting then on the last B node

    <xsl:variable name="nextB" select="number(following-sibling::B)"/>
following-sibling::B is empty so number is not-a-number (NaN)

David

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