Incrementer in Nested for-each-group Calls - xslt

Question

First post here after looking at tons of awesome suggestions from the community.
I have three fields in XSLT 2.0, all at the same level (shoulders, knees, and toes). I am needing to output sums of toes based on unique combinations of shoulders and knees, so I have created two nested for-each-groups. On each output, I'm also needing to output an incrementer from from 1 to number of unique combinations of shoulders and knees.
This incrementer is where I'm having issues. The closest I've come is by calling position(), but if I call it in the innermost group, the counter resets at each unique shoulder. If I call it in the outermost group, every knee inside of a unique shoulder gets the same value, then it resets at each unique shoulder. If I call it outside of the groups completely, it never gets past 1. I've also tried to use xsl:number , keys, etc., to no avail. In those cases, the correct number of rows are still being printed, but the incrementer values are looking at the individual, non-grouped values.
I read one suggestion about "tunneling" values between templates, but I haven't been able to get that to work, mostly because I don't think I'm invoking the templates correctly (with these fields being same-level and not parent-child). Any thoughts on making this work with for-each-group or otherwise? Many thanks in advance.
Sample XML:
<bodies>
<parts>
<shoulders>shoulders1</shoulders>
<knees>knees1</knees>
<toes>1</toes>
</parts>
<parts>
<shoulders>shoulders2</shoulders>
<knees>knees2</knees>
<toes>2</toes>
</parts>
<parts>
<shoulders>shoulders1</shoulders>
<knees>knees2</knees>
<toes>10</toes>
</parts>
<parts>
<shoulders>shoulders2</shoulders>
<knees>knees1</knees>
<toes>10</toes>
</parts>
<parts>
<shoulders>shoulders1</shoulders>
<knees>knees1</knees>
<toes>9</toes>
</parts>
<parts>
<shoulders>shoulders2</shoulders>
<knees>knees2</knees>
<toes>8</toes>
</parts>
</bodies>
Sample XSLT:
<xsl:stylesheet exclude-result-prefixes="xsl" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0" xmlns:this="urn:this-stylesheet" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsl:template match="/">
<xsl:for-each-group select="bodies/parts" group-by="shoulders">
<xsl:for-each-group select="current-group()" group-by="knees">
<xsl:value-of select="shoulders"/>
<xsl:text>, </xsl:text>
<xsl:value-of select="knees"/>
<xsl:text>, </xsl:text>
<xsl:value-of select="sum(current-group()/toes)"/>
<xsl:text>, </xsl:text>
<xsl:value-of select="position()"/>
<xsl:text>. </xsl:text>
</xsl:for-each-group>
</xsl:for-each-group>
</xsl:template>
</xsl:stylesheet>
Resulting Output:
shoulders1, knees1, 10, 1. shoulders1, knees2, 10, 2. shoulders2, knees2, 10, 1. shoulders2, knees1, 10, 2.
Desired Output:
shoulders1, knees1, 10, 1. shoulders1, knees2, 10, 2. shoulders2, knees2, 10, 3. shoulders2, knees1, 10, 4.

Answer 1

When you've got two nested xsl:for-each-group instructions like this, then an alternative is to do single level grouping on a composite key, like this:
<xsl:for-each-group select="bodies/parts" group-by="concat(shoulders, '~', knees)">
The position() will then increment the way you are looking for, if I've understood the requirement correctly.
This doesn't work, of course, if you actually want to produce hierarchically structured output in which the outer group is significant.

Answer 2

I've tried this three times now and I think you have to process your results and then count them. I've run out of time to try and figure a way to do the counting inline, because I think it can't be done.
<xsl:stylesheet exclude-result-prefixes="xsl" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" version="2.0" xmlns:this="urn:this-stylesheet" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
<xsl:template match="/">
<xsl:variable name="results" as="xsd:string*">
<xsl:for-each-group select="bodies/parts" group-by="shoulders">
<xsl:for-each-group select="current-group()" group-by="knees">
<xsl:value-of>
<xsl:value-of select="shoulders"/>
<xsl:text>, </xsl:text>
<xsl:value-of select="knees"/>
<xsl:text>, </xsl:text>
<xsl:value-of select="sum(current-group()/toes)"/>
<xsl:text>,</xsl:text>
</xsl:value-of>
</xsl:for-each-group>
</xsl:for-each-group>
</xsl:variable>
<xsl:for-each select="$results">
<xsl:value-of select=".,position()"/>
<xsl:text>. </xsl:text>
</xsl:for-each>
</xsl:template>
</xsl:stylesheet>