What is the ground that is not unpaved made of?
The ground is made of brick.
    src        edge_attr  dst
0     0               of    1
1     1      in front of    3
2     1   to the left of    2
3     1  to the right of    6
4     2  to the right of    1
5     3           behind    1
6     4               of    1
7     4  to the right of    6
8     4   to the left of    7
9     6   to the left of    4
10    6   to the left of    1
11    7  to the right of    4