What are the leaves on?
The leaves are on the tree.
    src        edge_attr  dst
0     0   to the left of   15
1     0   to the left of   11
2     0   to the left of   20
3     0  to the right of   18
4     1               on   22
..  ...              ...  ...
57   20  to the right of    0
58   20  to the right of   12
59   20  to the right of    6
60   20  to the right of   18
61   21   to the left of   10

[62 rows x 3 columns]