Do you see fences there that are metallic?
Yes, there is a metal fence.
    src        edge_attr  dst
0     0   to the left of    3
1     0   to the left of    9
2     3  to the right of    6
3     3   to the left of    9
4     3  to the right of    0
5     4               on    3
6     4  to the right of    6
7     4   to the left of    9
8     6   to the left of   13
9     6   to the left of    9
10    6   to the left of    4
11    6   to the left of    3
12    9  to the right of    6
13    9  to the right of    3
14    9  to the right of    0
15    9  to the right of    4
16   11               on   12
17   13  to the right of    6