a(7)=6:
._____________. ._____________. ._____________.
| |_________| | | | | | | | | | |_________| | |
| .___| |___. | | | | | | | | | |_________| | |
|_| ._| |_. |_| | | | | | | | | |_________| | |
| ._|_____|_. | | | | | | | | | |_________| | |
|_|_________|_| |_|_|_|_|_|_|_| |_________|_|_|
._____________. ._____________. ._____________.
| |_________| | | |_________| | | | |_________|
|_. |_. ._| ._| | |_________| | | | |_________|
| |___| |___| | | |_________| | | | |_________|
| .___|_|___. | | |_________| | | | |_________|
|_|_________|_| |_|_________|_| |_|_|_________| .