Where are the trees?
The trees are on the hills.
    src        edge_attr  dst
0     0   to the left of   12
1     2   to the left of    4
2     2   to the left of    9
3     4  to the right of    6
4     4  to the right of    2
5     6               on   11
6     6   to the left of    4
7     6   to the left of    9
8     6   to the left of    7
9     6               on    3
10    6   to the left of   12
11    7   to the left of    9
12    7  to the right of    6
13    9  to the right of    2
14    9  to the right of    7
15    9  to the right of    6
16   12               on    9
17   12  to the right of    6
18   12  to the right of    0