Do you see both numbers and letters?
Yes, there are both letters and numbers.
    src        edge_attr  dst
0     0  to the right of   15
1     4               in    2
2     9               on    3
3    10  to the right of   15
4    12               in    4
5    12  to the right of   15
6    14  to the right of   15
7    15   to the left of   10
8    15   to the left of   14
9    15   to the left of   12
10   15   to the left of    0