title: QueryDSL 4 select + where + exists
body: <p>Just started using queryDSL at my job. So far I think the syntax is very intuitive.</p>
  
  <p>I need to convert a very simple SQL query to queryDSL, but I don't know what do do with the EXISTS clause.</p>
  
  <pre><code>SELECT * FROM XRDocument document
   WHERE status = 0
     AND EXISTS
       (
         select * FROM XEntity X WHERE X.pid in (11,22,33)
       )
    AND NOT EXISTS
       (
        select * FROM XEntity X WHERE X.pid in (44,55,66)
       )
  </code></pre>
  
  <p>I have all my Q classes ready.
  So far this is all I have (pseudo code):</p>
  
  <pre><code>JPAQuery query = new JPAQuery(em);
  query.from(xDocument).where(xDocument.status.eq(0))
  </code></pre>
  
  <p>I read the documentation but there's no ".exists()" method. I also tried using the JDOExpressions class but I couldn't come up with a solution.
  Can anybody point me to the right direction?</p>
  
  <p>Thank you!</p>