title: Where would "abstract override" in subtrait with no implementation be useful, if any?
body: <p>Given the following example of two traits with one extending another with <em>no implementation</em> of <code>def a</code> in each:</p>
  
  <pre><code>scala&gt; trait A { def a: String }
  defined trait A
  
  scala&gt; trait B extends A { abstract override def a: String }
  defined trait B
  </code></pre>
  
  <p>Is the construct useful at all? What are the use cases?</p>