Group the rows using the composite key ARPT__IDENT | TRM__IDENT and store

<xsl:variable name="groups" as="array(element(row))*" select="

let $keys :=

distinct-values($rows/concat(ARPT__IDENT, '|', TRM__IDENT))

return

for $i in $keys

return

array {$rows[$i = concat(ARPT__IDENT, '|', TRM__IDENT)] }

Note that you can also create such a sequence of arrays where each array
contains the items belonging to a group with

B B B B B B B  <xsl:variable name="groups" as="array(element(row))*">
B B B B B B B B B B B  <xsl:for-each-group select="$rows" composite="yes"
group-by="ARPT__IDENT, TRM__IDENT">
B B B B B B B B B B B B B B B  <xsl:sequence select="array { current-group()
}"/>
B B B B B B B B B B B  </xsl:for-each-group>
B B B B B B B  </xsl:variable>

That will probably perform better than the pure XPath expression based
grouping (that does nothing more than apply the old XQuery 1.0
"grouping" technique to collect distinct-values first as a means to
select items of each group).