[xsl] xsl:for-each-group: start groups depending on number of group members?

[Yves Forkl <Y.Forkl@xxxxxx>]

Relying heavily on xsl:for-each-group in the context of rather 
complicated transformations, I have run into this situation (and am 
stuck there...): When specifying, in group-starting-with, the pattern 
that may start some specific group, I would like to take into account 
the number of items available for grouping.

More precisely, my group starting pattern must include a filter using 
preceding-sibling which I do not want to "look beyond" the current 
sequence of items to be grouped.

Showcasing this in a minimal example (more details below):

INPUT:

<root>
  <A/>
  <sub>a1</sub>
  <B/>
  <sub>b1</sub>
  <sub>b2</sub>
  <sub>b3</sub>
  <C/>
  <sub>c1</sub>
  <sub>c2</sub>
</root>

OUTPUT:

<B_new>
   <sub_new>b1</sub_new>
   <sub_new>b2</sub_new>
   <sub_new>b3</sub_new>
</B_new>

STYLESHEET:

<xsl:stylesheet xmlns:xsl="http://www.w3.org/1999/XSL/Transform";
                version="2.0">

  <xsl:output indent="yes"/>

  <xsl:template match="/">
    <xsl:for-each-group
      select="root/*"
      group-starting-with="A|B|C">
      <xsl:apply-templates
        select="current-group()[1]"
        mode="groups_at_root_level"/>
    </xsl:for-each-group>
  </xsl:template>

  <!-- don't care for them now -->
  <xsl:template match="A|C" mode="groups_at_root_level"/>

  <xsl:template match="B" mode="groups_at_root_level">
    <B_new>
      <xsl:variable name="items_total" select="count(current-group())"/>
      <xsl:for-each-group
        select="current-group()"
        group-starting-with="
          B|
          sub[not(
            preceding-sibling::*[position() lt $items_total][A])]">
        <xsl:apply-templates select="current-group()"/>
      </xsl:for-each-group>
    </B_new>
  </xsl:template>

  <xsl:template match="sub">
    <sub_new>
      <xsl:apply-templates/>
    </sub_new>
  </xsl:template>

  <xsl:template match="*"/>

</xsl:stylesheet>


I would like to ask your opinion on the preceding-sibling line. If it 
just read preceding-sibling::A, the output would of course miss all 
"sub" instances. (Unfortunately, for reasons difficult to explain here, 
I can't just drop the negated preceding-sibling filter because I need it 
in some of my group starting patterns.)

Obviously, my above try at restricting the number of preceding siblings 
to consider to those which are among the items to group is rather 
limited: When xsl:for-each-group examines the very first item, the first 
preceding sibling of that will already be outside of the groupable items 
(while the last item will have many more "acceptable" preceding siblings 
than the first). This makes me dream of a kind of dynamic position 
counter returning the position of the current item that 
xsl:for-each-group is trying to put into a group. Is there such a thing?

Alternatively, how could I restrict sibling tests to the items which are 
being grouped, inside the pattern of group-starting-with?

  Yves