intro:
|----------------------------------------|
|----------------------------------------|
|-2--4-5----2--4-5----2--4-5----4-5-7-9\-|
|----------------------------------------| 
|----------------------------------------|
|----------------------------------------|
                                . . .

|--------------------------------------------|
|--------------------------------------------|
|---------------------------------------7-9\-|
|-7--9-10----7--9-10----7--9-10----9-10-7-9\-| 
|-7--9-10----7--9-10----7--9-10----9-10-5-7\-|
|-5--7-8-----5--7-8-----5--7-8-----7-8-------|

|----------------------------------------|
|----------------------------------------|
|-2--4-5----2--4-5----2--4-5----4-5-7-9\-|
|----------------------------------------| 
|----------------------------------------|
|----------------------------------------|
                                  . . .

|--------------------------------------------|
|--------------------------------------------|
|---------------------------------------7-9\-|
|-7--9-10----7--9-10----7--9-10----9-10-7-9\-| 
|-7--9-10----7--9-10----7--9-10----9-10-5-7\-|
|-5--7-8-----5--7-8-----5--7-8-----7-8-------|