What's under the bridge?
The river is under the bridge.
   src        edge_attr  dst
0    0  to the right of    3
1    2  to the right of    7
2    2  to the right of    9
3    3   to the left of    0
4    7   to the left of    2
5    9   to the left of    2
6   10            under   12
7   11       walking on   12
8   12            above   10