What's under the bridge?
The boats are under the bridge.
    src        edge_attr  dst
0     1               on   15
1     2               on   14
2     3  to the right of   14
3     3  to the right of    7
4     7   to the left of    3
5     7   to the left of   14
6     8               on   16
7     8            under   15
8     8               in   16
9     8            below   15
10   14   to the left of    3
11   14  to the right of    7
12   15            above    8