- mainQ [(1), (2), (3)]  --- CONCURRENT_QUEUE(並列キュー)
- NCQ(singleton)  []     --- SERIAL_QUEUE(直列キュー)

```
-----------------------------
自分らが思ってた動き
-----------------------------
mainQ(1)
↓
NCQ [(1a), (1b), (1c)]
↓
mainQ(2)
↓
NCQ [(1a), (1b), (1c), (2a), (2b), (2c)]
↓
mainQ(3)
↓
NCQ [(1a), (1b), (1c), (2a), (2b), (2c), (3a), (3b), (3c)]


-----------------------------
でも、実際はmainのQueueが並列なので
-----------------------------

mainQ(1) ------>  mainQ(2) --------> mainQ(3)
↓                  ↓                  ↓
NCQ[(1a)]          ↓                  ↓
                   NCQ[(1a), (2a)]    ↓
                                    NCQueue[(1a), (2a), (3a)]
↓                  ↓                  ↓
NCQ[(1a), (2a), (3a), (1b)]           ↓
                   ↓                  ↓
                  NCQ[(1a), (2a), (3a), (1b), (2b)]
                                      ↓
                                    NCQ[(1a), (2a), (3a), (1b), (2b), (3b)]

```