1.0
 |
1.1
 |
1.2 ---\  ------\
 |      \        \
1.3   1.2.0   1.2.0.0
 |      |       
1.4   1.2.1 ----\
 |      |        \
1.5   1.2.2   1.2.1.0 
 |               |
1.6           1.2.1.1