What is covering the ground?
The leaves are covering the ground.
    src        edge_attr  dst
0     0   to the left of    4
1     0   to the left of    1
2     1  to the right of    3
3     1  to the right of    5
4     1  to the right of    4
5     1  to the right of    0
6     2   to the left of    7
7     3   to the left of    4
8     3   to the left of    1
9     4  to the right of    0
10    4   to the left of    1
11    4  to the right of    3
12    5   to the left of    1
13    5   to the left of    8
14    5   to the left of    7
15    6       covered by    7
16    7  to the right of    2
17    7         covering    6
18    7  to the right of    5
19    8  to the right of    5