I've seen from a cursory glance on the repo that a lot of people
struggle with this problem set because of the ambiguity in the
instructions (See issues #287, #743, #567).
I'd like to recommend perhaps putting a TODO on the lines that need
changing to stop people from falling down rabbit holes and pursuing
completely different solution to the exercise that avoids the core
pedagogical content of the problem, which is an understanding of
how Arc is an atomic reference counting primitive that relies on
.lock() and .unwrap() to solve shared state concurrency problems
with the Mutex primitive.
I've also added comments in the instructions to highlight why it is that
the solution is solved when the program prints the counter to the screen
6 times, as this was unclear and was distracting students from the core
takeaway of the exercise.
3e6a8cfbdb