A simple function

search-function.xql

xquery version "3.0";

declare namespace tei="http://www.tei-c.org/ns/1.0";
declare namespace ft="http://exist-db.org/xquery/lucene";

declare function local:search($terms as xs:string*) as node(){
    if(starts-with($terms,'"')) then
    <query>
        <phrase>{$terms}</phrase>
    </query>  
    else 
    <query>
        <term>{$terms}</term>
    </query>
};

let $term :=  '"et tu"'
let $query := local:search($term)
for $hits in collection('/db/apps/demo-app/data')//tei:sp[ft:query(.,$query)]
return $hits