title: Swift Programming language, what does it mean to start an attribute with a period
body: <p>For example, in the following line of code:</p>
  
  <pre><code>DispatchQueue.global(qos: .userInitiated).async { ... }
  </code></pre>
  
  <p>Why does <code>.userInitiated</code> start with a period? Does this follow from Objective C or is it specific to Swift?</p>