title: What is a sealed trait?
body: <p>Sealed classes are described in 'Programming in Scala', but sealed traits are not.
  Where can I find more information about a sealed trait?</p>
  
  <p>I would like to know, if a sealed trait is the same as a sealed class?
  Or, if not, what are the differences? 
  When is it a good idea to use a sealed trait (and when not)?</p>