title: Scala.NotImplementedError: an implementation is missing?
body: <p>Here is my code:</p>
  
  <pre><code>package example
  
  object Lists {
  
    def max(xs: List[Int]): Int = {
      if(xs.isEmpty){
          throw new java.util.NoSuchElementException()
      }
      else {
          max(xs.tail)
      }
    }
  }
  </code></pre>
  
  <p>When i run it in sbt console:</p>
  
  <pre><code>scala&gt; import example.Lists._
  scala&gt; max(List(1,3,2))
  </code></pre>
  
  <p>I have the following error:</p>
  
  <pre><code>Scala.NotImplementedError: an implementation is missing
  </code></pre>
  
  <p>How can I fix that?</p>
  
  <p>Thanks.</p>