What are the pieces of furniture that are filled with books?
The pieces of furniture are shelves.
    src        edge_attr  dst
0     0           behind    1
1     1      in front of    0
2     2  to the right of    4
3     3      filled with    2
4     3      filled with    4
5     4   to the left of    2
6     4   to the left of    7
7     4               on    3
8     5   to the left of    6
9     6  to the right of    5
10    7  to the right of    4