How long is the dark tunnel?
The tunnel is long.
    src        edge_attr  dst
0     0               on    3
1     1  to the right of    5
2     1   to the left of    4
3     2   to the left of    4
4     3   to the left of    4
5     3   to the left of    6
6     3   to the left of    9
7     4  to the right of    1
8     4  to the right of    2
9     4  to the right of    3
10    4  to the right of    8
11    5   to the left of    1
12    5               on    2
13    5   to the left of    8
14    6   to the left of    9
15    6  to the right of    7
16    6  to the right of    3
17    7   to the left of    6
18    7   to the left of    9
19    8  to the right of    5
20    8   to the left of    4
21    9  to the right of    3
22    9  to the right of    7
23    9  to the right of    6