Turns out the initial trial of using
Manhattan distance was required to
solve part 2, since the BFS
implementation is impossible to
scale up from 2 robots to 25.
Recursion and memoization makes the
execution time acceptable.
Line 53 makes all the difference here.
The order of `<|v|^|>` matters when
constructing a sequence. Many
hours was spent trying to find the correct
priority. For the example input, especially
456A and 379A were volatile.
