What's the pen made of?
The pen is made of concrete.
    src        edge_attr  dst
0     0  to the right of   16
1     0               of   10
2     0  to the right of   18
3     2   to the left of    9
4     2   to the left of    7
5     2   to the left of    6
6     2   to the left of   11
7     2   to the left of   13
8     3   to the left of   15
9     5  to the right of   18
10    5  to the right of   15
11    5  to the right of   10
12    5  to the right of    8
13    5  to the right of   12
14    6  to the right of    2
15    6  to the right of   13
16    6  to the right of    9
17    6  to the right of    7
18    7  to the right of    2
19    7   to the left of    6
20    8   to the left of    5
21    9   to the left of    6
22    9  to the right of    2
23   10               in   17
24   10   to the left of    5
25   10               of   10
26   11  to the right of   13
27   11  to the right of    2
28   12   to the left of    5
29   12   to the left of   18
30   12               of   10
31   13   to the left of    6
32   13  to the right of    2
33   13   to the left of   11
34   15   to the left of    5
35   15  to the right of    3
36   16   to the left of    0
37   16  to the right of   18
38   18   to the left of    0
39   18   to the left of    5
40   18               of   10
41   18  to the right of   12
42   18   to the left of   16