Have a bunch of code to temporarily chop out because of reasons? In C you could use
#if 0to tear out chunk of your file.
#ifin Swift generally requires the guts of the conditional to be syntactically correct (even if it's semantically nonsense).
You can abuse the Swift version check to do something similar to
#if 0:... #if swift(>=666) aj2n42j4n23jnjsdnfjsnfjnunrun unr unwu nudjfn jsnf jn var window: UIWindow? #endif ...The compiler still does some processing of the code, so you might get an error (like if that cat-stomp at the beginning started with a digit, Swift tries interpreting 2n42j4n23jnjsdnfjsnfjnunrun as an integer literal and fails)
(Thanks to Jeremy Sherman for the idea)