title: var in protocol as specific protocol
body: <p>How to describe a variable in a protocol on swift
  any type, but supports a specific protocol</p>
  
  <p>Something like this</p>
  
  <pre><code>protocol MyProtocol: class {
      var itemsList: AnyObject where Collection { get } // AnyObject supports a Collection protocol
  }
  </code></pre>