             operation.addCompletionBlock { [weak self, weak operation] in
                 guard let queue = self, let operation = operation else { return }
                 queue.delegate?.operationQueue?(queue, operationDidFinish: operation, withErrors: [])
-                queue.ops.remove(operation)
+                queue.removeOpFromSet(operation)
             }
         }
         

    - Setting up dependencies to enforce mutual exclusivity
*/
public class OperationQueue: NSOperationQueue {
    private var ops: Set<NSOperation> = Set()
    public weak var delegate: OperationQueueDelegate?
    
    override public  func addOperation(operation: NSOperation) {
        ops.insert(operation)
        if let op = operation as? Operation {
            
            // Set up a `BlockObserver` to invoke the `OperationQueueDelegate` method.

            }
        }
    }
    
    func addOpToSet(op: NSOperation) {
        dispatch_async(opsQueue) {
            self.ops.insert(op)
        }
    }
    
    func removeOpFromSet(op: NSOperation) {
        dispatch_async(opsQueue) {
            self.ops.remove(op)
        }
    }
}
