a(3)=7:
._____. ._____. ._____. ._____. ._____. ._____. ._____.
| | | | | | | | | | | | | | | | |_. | | ._|
| | | | | | ._| | |_. | | ._| | |_. | | | |___| |___| |
| | | | | |_| | | | |_| |_| | | | |_| | |___. | | .___|
| | | | | | | | | | | | | | | | | |_| |_| |
|_|_|_| |_|___| |_|___| |___|_| |___|_| |_____| |_____| .