What is the vehicle that is behind the boy the cap is on?
The vehicle is a bus.
    src        edge_attr  dst
0     0   to the left of   13
1     0   to the left of    6
2     4   to the left of   18
3     4   to the left of    6
4     5   to the left of   12
5     6  to the right of    0
6     6  to the right of    8
7     6  to the right of    4
8     6  to the right of    9
9     7           behind   17
10    8   to the left of   18
11    8   to the left of    6
12    9   to the left of    6
13   12  to the right of    5
14   13  to the right of    0
15   17          wearing    0
16   17        riding on   18
17   17          wearing    5
18   17      in front of    7
19   18  to the right of    8
20   18  to the right of    4