title: What does "get" mean in a protocol's property declaration?
body: <p>I'm looking at some code from an auto layout library. In it, there is a protocol adopted by UIView:</p>
  
  <pre><code>extension UIView: Constrainable {}
  extension UILayoutGuide: Constrainable {
      // LayoutGuide doesn't have baseline anchors, so just use the bottom anchor
      public var firstBaselineAnchor: NSLayoutYAxisAnchor {
          return bottomAnchor
      }
      public var lastBaselineAnchor: NSLayoutYAxisAnchor {
          return bottomAnchor
      }
  }
  
  public protocol Constrainable {
      var topAnchor:      NSLayoutYAxisAnchor { get }
      var bottomAnchor:   NSLayoutYAxisAnchor { get }
      var leftAnchor:     NSLayoutXAxisAnchor { get }
      var rightAnchor:    NSLayoutXAxisAnchor { get }
      var leadingAnchor:  NSLayoutXAxisAnchor { get }
      var trailingAnchor: NSLayoutXAxisAnchor { get }
  
      var centerXAnchor:  NSLayoutXAxisAnchor { get }
      var centerYAnchor:  NSLayoutYAxisAnchor { get }
  
      var widthAnchor:    NSLayoutDimension { get }
      var heightAnchor:   NSLayoutDimension { get }
  
      var firstBaselineAnchor : NSLayoutYAxisAnchor { get }
      var lastBaselineAnchor  : NSLayoutYAxisAnchor { get }
  }
  </code></pre>
  
  <p>What does an empty <code>{ get }</code> accomplish? </p>