What is the blue sign mounted on?
The sign is mounted on the cart.
    src         edge_attr  dst
0     0   to the right of    5
1     0    to the left of    2
2     1                on    4
3     1   to the right of    5
4     2   to the right of    5
5     2   to the right of    0
6     4        mounted on    7
7     4   to the right of    5
8     5    to the left of    2
9     5    to the left of    7
10    5  standing next to    0
11    5    to the left of    4
12    5    to the left of    1
13    5    to the left of    0
14    6    to the left of    7
15    7   to the right of    6
16    7   to the right of    5