One final update .. I don't know if other environments behave differently,
but with Filemaker's XML/XSL export I had to change this line:

>   <xsl:variable name="this-role"><xsl:number/></xsl:variable>

to this:

>   <xsl:variable name="this-role"><xsl:number level="single"
>       count="fm:DATA"/></xsl:variable>

otherwise <xsl:number/> was producing "" (and number($this-role) was
producing NaN).

e.