Is this a train or a bus?
This is a train.
   src        edge_attr  dst
0    0  to the right of    3
1    1  to the right of    4
2    1  to the right of    6
3    3   to the left of    0
4    4   to the left of    6
5    4   to the left of    1
6    6  to the right of    4
7    6   to the left of    1