I don't understand the reason behind this example? It boils down to "I did a read-modify-write without a lock, now there is a bug". Of course there is! that's

I don't understand the reason behind this example? It boils down to "I did a read-modify-write without a lock, now there is a bug". Of course there is! that's what locks are for! The swift value system and copy-on-write do not somehow absolve you from locking.