title: Conversion from Seq to Set and back to Seq
body: <p>Intuitively the following should work:</p>
  
  <pre><code>case class I(i: Int)
  val s = Seq(I(1),I(2),I(3))
  s.sortBy(_.i)             // works
  s.toSeq.sortBy(_.i)       // works
  s.toSet.toSeq.sortBy(_.i) // doesn´t work
  </code></pre>
  
  <p>Why doesn´t it behave as expected?</p>