tion.  The result can only be retrieved using method
 <code>get</code> when the computation has completed, blocking if
 necessary until it is ready.  Cancellation is performed by the
 <code>cancel</code> method.  Additional methods are provided to
 determine if the task completed normally or was cancelled. Once a
 computation has completed, the computation cannot be cancelled.
 If you would like to use a <code>Future</code> for the sake
 of cancellability but not provide a usable result, you can
 declare ty