title: Underlying type for Tuple in Swift
body: <p>What's the underlying type for Tuple in Swift? I see one mention of Tuple in the Swift module but I can't any of the following:</p>
  
  <pre><code>var x: Tuple
  var y: Optional&lt;Tuple&gt;
  </code></pre>
  
  <p>Is Tuple a compiler magic or an actual type in Swift?</p>