,---------------.
|B              |
|---------------|
|Level 1        |
||_ Level 2a    |
|  |_ Level 3a  |
|  |_ Level 3b  |
|  |_ Level 3c  |
|    |_ Level 4a|
|  |_ Level 3d  |
||_ Level 2b    |
|  |_ Level 3e  |
`---------------'