What animals are old?
The animals are horses.
    src        edge_attr  dst
0     1   to the left of    2
1     2  to the right of    1
2     4  to the right of   16
3     4  to the right of    9
4     4  to the right of    6
5     4  to the right of   19
6     4  to the right of   17
7     6   to the left of    4
8     6   to the left of   13
9     7      standing in    3
10    7   to the left of   14
11    7  to the right of   13
12    9   to the left of   13
13    9   to the left of    4
14   10      standing in    3
15   10  to the right of   13
16   10   to the left of   14
17   11   to the left of   14
18   11  to the right of   13
19   13  to the right of    6
20   13      standing in    3
21   13   to the left of   11
22   13   to the left of    7
23   13  to the right of   16
24   13  to the right of   19
25   13  to the right of   17
26   13  to the right of    9
27   13   to the left of   10
28   14  to the right of   11
29   14      standing in    3
30   14  to the right of    7
31   14  to the right of   10
32   16   to the left of   13
33   16   to the left of    4
34   17   to the left of    4
35   17   to the left of   13
36   17   to the left of   19
37   19   to the left of    4
38   19   to the left of   13
39   19  to the right of   17
40   20               in   15