try? isLowerHalf(70) // nil
try? isLowerHalf(20) // 20

do {
    let value = try isLowerHalf(70)
    print("Yes, \(value) is in the lower half!")
} catch is NotLowerHalf {
    print("Not in the lower half.")
}

if let value = try? isLowerHalf(20) {
    print("OK, that worked, and it's not _too_ weird.")
}

let result = try! isLowerHalf(40) // would cause a crash if it failed