Which type of material is the shelf to the left of the other shelf made of?
The shelf is made of wood.
    src        edge_attr  dst
0     0   to the left of   13
1     0               on    7
2     1   to the left of    4
3     1  to the right of   18
4     3  to the right of   18
5     4  to the right of   10
6     4  to the right of    1
7     4  to the right of   18
8     4  to the right of   17
9     5   to the left of   18
10    5   to the left of    7
11    6   to the left of   12
12    7  to the right of    5
13    7  to the right of   16
14    8   to the left of   15
15    9  to the right of   17
16    9               in    1
17   10   to the left of    4
18   12  to the right of    6
19   13  to the right of    0
20   15  to the right of    8
21   16   to the left of    7
22   16   to the left of   17
23   17   to the left of    4
24   17   to the left of    9
25   17  to the right of   16
26   18   to the left of    3
27   18   to the left of    4
28   18  to the right of    5
29   18   to the left of    1