Thanks Gerrit,
This got me going.

>>        <xsl:for-each-group select="ss:Row"
>> group-starting-with="ss:Cell[1][not(@ss:Index)]">
>
> This is a pattern. You have to specify at which element the grouping should
> start, just as you'd specify a template's match:
>  group-starting-with="ss:Row[not(ss:Cell[1]/@ss:Index)]">

I someone imagined that the context of group-starting-with was a child
of xsl:for-each-group.

Fred