On which kind of vehicle is the ladder?
The ladder is on the truck.
    src        edge_attr  dst
0     0   to the left of    1
1     0               of    3
2     1  to the right of    0
3     2   to the left of    6
4     2  to the right of    5
5     4  to the right of    5
6     4   to the left of    6
7     5   to the left of    2
8     5   to the left of    6
9     5   to the left of    4
10    6  to the right of    5
11    6               on    3
12    6  to the right of    4
13    6  to the right of    2