   #   to ensure sequential processing for all tasks of the same group (aka.
   #   first-come-first-serve). Current time is appended at the argument array
   #   for statistics. A callback is optional.
+  #
+  # **Usage:**
   #
   #     queue.enqueue 'param1', 'param2', -> console.log 'queued!'
   #     queue.enqueue 'param1', 'param2'


  # ### Poll New Task

  # If any task presents in the `SOURCE` list, `lpop` from `SOURCE`
  # (`rpush`) into `QUEUED`. The `lpop` and `rpush` are protected by a
  # transaction.
  #
  # Since the task being popped and pushed should be known in prior of the
  # begin of the transaction (aka, the `multi` command), we need to get the

  # ### Get Currently Processing Tasks Asynchronously
  #
  # Currently processing tasks are tasks in the `RECENT` list, which will be
  # limited to a maximum size defaults to 10. **Usage:**
  #
  #     queue.processing_tasks (tasks) ->
  #       console.log "#{tasks.length} tasks being processing: ", tasks
