     for pair in numberArray {
       let operation = SumOperation(input: pair)
       operation.completionBlock = {
-        guard let result = operation.output else { return }
-        self.outputArray.append((pair.0, pair.1, result))
+        // TODO: Implement this
       }
       queue.addOperation(operation)
     }

}

//: `GroupAdd` is a vanilla class that manages an operation queue and multiple `SumOperation`s to calculate the sum of all the pairs in the input array.
class GroupAdd {
  let queue = NSOperationQueue()
  var outputArray = [(Int, Int, Int)]()

  
  func start() {
    // TODO: implement this
  }
  
  func cancel() {
    // TODO: implement this
  }
  
  func wait() {
