title: Ensure list contains only specific values
body: <p>How can I make sure a list only contains a specific set of items?</p>
  
  <pre><code>List[Int]
  </code></pre>
  
  <p>A function to make sure the list only contains the values 10, 20 or 30.
  I'm sure this is built in by I can't find it!</p>