return callable.call();
          }

          @Override
          public String toString() {
            return callable.toString();
          }
        };
    /*
     * Four futures are at play here:
     * taskFuture is the future tracking the result of the callable.
     * newFuture is a future that completes after this and all prior tasks are done.
     * oldFuture is the previous task's newFuture.