What is the shelf made of?
The shelf is made of metal.
    src        edge_attr  dst
0     1   to the left of   14
1     1  to the right of   12
2     3   to the left of   14
3     3  to the right of    9
4     4  to the right of    5
5     5   to the left of    6
6     5   to the left of    4
7     5   to the left of   12
8     6  to the right of    5
9     7  to the right of   11
10    9   to the left of    3
11   10  to the right of   13
12   11  to the right of   13
13   11   to the left of    7
14   12  to the right of    5
15   12   to the left of    1
16   12   to the left of   14
17   12               in   11
18   13   to the left of   11
19   13   to the left of   10
20   14  to the right of    1
21   14  to the right of   12
22   14  to the right of    3