> Here is the missing code :

ah, which is more or less the same as I came up with, modulo a few
variables,

The xslt2 solution, using xsl:for-each-group, would be essentially
identical to this just doing a straight translation of

<xsl:for-each select="salesman/account[count( . |...

to xsl:for-each-grouo, and

            <xsl:for-each select="key('accounts-by-region', $region)">

to 

<xsl:for-each select="current-group()"

so it's a fair bit more readable but structurally identical.

David