title: firebase swift 3 conversion : Cannot convert value of type '(_) -> ()' to expected argument type '((Error) -> Void)?
body: <p>I convert my code to swift 3 , and Im getting the following error when trying to read some data:</p>
  
  <p>Cannot convert value of type '(_) -> ()' to expected argument type '((Error) -> Void)?'</p>
  
  <pre><code>Ref.observe(.value, with: { snapshot in
     print(snapshot)
  }, withCancel: { error in  //the error appear here
          print(error.description)
  })
  </code></pre>