 
     /**
      * Flexible asynchronous tasker.
+     * @see http://docs.tatsuya-koyama.com/krew-framework/samples/krewasync/
      *
      * Usage:
      * <pre>

     *             ]},
     *             function_7
     *         ],
     *         error  : _onErrorHandler,
     *         anyway : _onFinallyHandler
     *     });

     *     [Sequence]:
     *                   |3 ------>|
     *                   |         |
     *         1 -> 2 -> |4 ------>| -> 7 -> success -> anyway
     *                   |         |        (or error)
     *                   |5 -> 6 ->|
     *
     * </pre>
