Dan Rosenstark, Author of MIDI Designer, muses about all things tech. Particularly: Notes on software development in Swift, Objective-C, and many non-Apple languages. Also: lots of random technology notes on OS X and iOS.
Even in a Playground it's easy to see how locks work. When useLocks is set to false, a fatal error is thrown before the counter even gets to 5! When it's set to true, the counter can get to its natural endpoint (200).