Is there a flag or a fence?
Yes, there is a fence.
    src        edge_attr  dst
0     0   to the left of    1
1     0   to the left of    2
2     0   to the left of    6
3     0   to the left of    3
4     1  to the right of    0
5     1  to the right of    9
6     2  to the right of    9
7     2   to the left of   11
8     2  to the right of    0
9     3  to the right of    0
10    3  to the right of    9
11    4  to the right of   11
12    6  to the right of    9
13    6  to the right of    0
14    7   to the left of   11
15    9   to the left of    2
16    9   to the left of    3
17    9   to the left of    6
18    9   to the left of    1
19   11   to the left of    4
20   11  to the right of    2
21   11  to the right of    7