What is the shelf made of?
The shelf is made of wood.
    src        edge_attr  dst
0     0   to the left of    1
1     0   to the left of    2
2     0  to the right of    3
3     1  to the right of    0
4     1               on    4
5     1  to the right of    5
6     1  to the right of    6
7     2  to the right of    5
8     2  to the right of    0
9     2               on    4
10    3               on    4
11    3   to the left of    0
12    3   to the left of    5
13    4      in front of    9
14    5               of    4
15    5   to the left of    2
16    5   to the left of    1
17    5  to the right of    3
18    6   to the left of    1
19    9           behind    4