What is in the sign?
The letter is in the sign.
    src        edge_attr  dst
0     1   to the left of   15
1     3   to the left of   10
2     5  to the right of   17
3     6      attached to    1
4     7   to the left of   10
5     9       printed on   13
6     9               in   13
7    10  to the right of    3
8    10  to the right of   15
9    10  to the right of    7
10   14   to the left of   15
11   14               of   14
12   15  to the right of   14
13   15  to the right of    1
14   15   to the left of   10
15   17   to the left of    5