What is the wall behind of, a truck or a train?
The wall is behind a train.
    src        edge_attr  dst
0     0   to the left of   11
1     0   to the left of    9
2     0   to the left of   21
3     0   to the left of   13
4     1  to the right of    9
..  ...              ...  ...
60   19   to the left of    9
61   20   to the left of    7
62   21  to the right of   13
63   21  to the right of    9
64   21  to the right of    0

[65 rows x 3 columns]