What type of vehicle is the tree behind of?
The tree is behind the bus.
    src        edge_attr  dst
0     1               on    6
1     2  to the right of    5
2     2  to the right of   10
3     2           behind    5
4     3  to the right of   10
5     3               on    4
6     3  to the right of    5
7     4  to the right of    5
8     4  to the right of    9
9     4  to the right of    7
10    5   to the left of    3
11    5   to the left of    2
12    5      in front of    2
13    5   to the left of    4
14    7   to the left of    4
15    8               of    5
16    9               of    5
17    9   to the left of    4
18   10   to the left of    3
19   10   to the left of    2