declare namespace request="http://exist-db.org/xquery/request";
declare namespace transform="http://exist-db.org/xquery/transform";

let $term := request:request-parameter("term","")
let $terms := collection('db/ChrisWallace/medical')/terms/term//def[. = $term]/ancestor::term
let $ss:= doc('/db/ChrisWallace/medical/terms.xsl')
let $params := 
<parameters>
  <param name="heading" value="Terms referencing {$term}"/>
</parameters>

return
   transform:transform(<Result>{$terms}</Result>,$ss,$params)