Something

module namespace tfi = "http://www.transfrance.info/";

declare namespace set = "java:java.util.TreeSet";
declare variable $tfi:set := set:new();

declare function tfi:test($element as xs:string) {
  let $void := set:add($tfi:set, $element)
  return set:size($tfi:set)
};


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import module namespace tfi = "http://www.transfrance.info/" at "...";

tfi:test('hello')