title: What is "Element" type?
body: <p>Reading <strong>Swift Programming Language</strong> book I've seen number of references to type <code>Element</code>, which is used to define type of collection items. However, I can't find any documentation on it, is it class, protocol? What kind of functionality/methods/properties it has?</p>
  
  <pre><code>struct Stack&lt;Element&gt;: Container {
      // original Stack&lt;Element&gt; implementation
      var items = [Element]()
      mutating func push(_ item: Element) {
          items.append(item)
      }
      ...
  </code></pre>