title: dispatch_once after the Swift 3 GCD API changes
body: <p>What is the new syntax for <code>dispatch_once</code> in Swift after the changes made in language version 3? The old version was as follows.</p>
  
  <pre><code>var token: dispatch_once_t = 0
  func test() {
      dispatch_once(&amp;token) {
      }
  }
  </code></pre>
  
  <p>These <a href="https://github.com/apple/swift-evolution/blob/master/proposals/0088-libdispatch-for-swift3.md" rel="noreferrer">are the changes to libdispatch</a> that were made. </p>