title: Foldable IntSet
body: <p>For me, an integer set seems to be a foldable data structure.
  Why is <code>Data.IntSet</code> not an instance of <code>Foldable</code>?</p>
  
  <p>My actual intention is to use <code>find</code> on an <code>IntSet</code>.
  How can I implement find for <code>Data.IntSet</code>?</p>