Interesting tangle I am in. I tried the map:merge in both the iterate
and apply-templates solutions. I can indeed reference the bpb and pull
it into the output.B However, it seems to play havoc with the $groups
output. Here is the changed iterate:

 B  B  B  B  B  B  B  B  B  B  B <xsl:iterateselect="p[@class = 'nl'] !
map:merge(($groups, map
 B  B  B  B  B  B  B  B  B  B  B  B  { 'p' : . }))">
<xsl:text>&#10;</xsl:text>
<write_choiceB  num="{position() - 1}"letter="{?letter}"
 B  B  B  B  B  B  B  B  B  B  B  B  B  B  term="{?term}"item="{?p}"/>
</xsl:iterate>

<xsl:apply-templates
        select="for $p in p[@class = 'nl']
                return $groups
                       !
                       map:merge(
                         ($groups,
                          map { 'p' : $p })
                       )"/>

$groups is a sequence of maps we want to process for each "p" and
including the "p" in each map of the sequence is what we need, not to
merge the sequence of maps.