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

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

  #
  # **Usage:**
  #
  #     queue.slowest_tasks (err, task) -> YOUR CODE
  #
  # `slowest_tasks` is an asynchronous method. The only arg of the callback
  # function is an array of slowest tasks in the reverse order by processing
