title: how to force method to be implemented in concrete subclass from trait
body: <p>I have a method in my trait</p>
  
  <pre><code>def controller: AnyRef
  </code></pre>
  
  <p>but my concrete class was not implementing that method and it was still compiling.  The compiler doesn't let me add abstract to that method either.  How can I create a method in a trait that forces it's implementer to implement it?</p>
  
  <p>thanks,
  Dean</p>