      Sends the a block to be run asynchronously on a queue with a quality of service of QOS_CLASS_UTILITY.
 
      - parameters:
-     - after: After how many seconds the block should be run.
-     - block: The block that is to be passed to be run on queue
+        - after: After how many seconds the block should be run.
+        - block: The block that is to be passed to be run on queue
 
      - returns: An `Async` struct
 

     Sends the a block to be run asynchronously on a queue with a quality of service of QOS_CLASS_BACKGROUND.

     - parameters:
     - after: After how many seconds the block should be run.
     - block: The block that is to be passed to be run on the queue

     - returns: An `Async` struct



    Usage:

        let block1 = Async.background {
            // Some work
        }
        let block2 = block1.background {
            // Some other work
        }
        Async.main {
            // Cancel async to allow block1 to begin
            block1.cancel() // First block is NOT cancelled
            block2.cancel() // Second block IS cancelled
        }

    */
    public func cancel() {
