declare namespace fun = 'have.more.fun';

declare function fun:closure($input as node()*, $result as node()*) as node()*
{
   let $current := $input/idref(.)[name() = "from"]/../@to
   let $new := $current except $result 
   let $all := ($result,$new) 
 
   return
      if(exists($new)) 
      then ($new, fun:closure($new,$all))
      else ()
};

doc()//category[@id="category0"]/@id/fun:closure(.,())/id(.)/name