title: Cannot assign to property: 'xxxx' is a get-only property
body: <p>I'm using a computed property to get the last book in my <code>books</code> array. However, it seems I can't use this property to directly set a book's <code>position</code> property, as my attempt below shows:</p>
  
  <pre><code>struct Book {
      var position: CGPoint?
  }
  
  class ViewController: UIViewController {
  
      var books = [Book]()
  
      var currentBook: Book {
          return books[books.count - 1]
      }
  
      func setup() {
          // Compiler Error: Cannot assign to property: 'currentBook' is a get-only property
          currentBook.position = CGPoint.zero
      }
  }
  </code></pre>
  
  <p>The following works but I'd like it to be more readable <em>and</em> a single line.</p>
  
  <pre><code>books[books.count - 1].position = CGPoint.zero
  </code></pre>
  
  <p>I could use a function to return the current book but using a property would be cleaner. Is there another way?</p>