   #     - poll task from the `SOURCE` queue.
   #   + **failed**, then inspect the passed in argument, retry or block
   #   according to the `do` property of the error object.
-  #
   #
   # Calling the callback function is the responsibility of you. Otherwise
   # `Fairy` will stop dispatching tasks.

  #
  # **Usage:**
  #
  #     queue.processing_tasks (tasks) -> YOUR CODE HERE
  processing_tasks: (callback) ->
    @redis.hvals @key('PROCESSING'), (err, res) ->
      return callback err if err

  #
  # **Usage:**
  #
  #     queue.failed_tasks (err, tasks) -> YOUR CODE
  failed_tasks: (callback) =>
    @redis.lrange @key('FAILED'), 0, -1, (err, res) ->
      return callback err if err
