enum Planet: Int {
    case mercury, venus, earth, mars, jupiter, saturn, uranus, neptune
    // case pluto

    static var gasGiants: Set<Planet> = [.jupiter, .saturn]
}

let planet: Planet = .jupiter

if Planet.gasGiants.contains(planet) {
    print("\(planet) is a gas giant")
}


// ============
// alternatively...
// this could be extended generically to all collections of any type,
// but that could cause hidden performance issue if `contains` is not O(1)
func ~=(planets: Set<Planet>, planet: Planet) -> Bool {
    return planets.contains(planet)
}

switch planet {
    case Planet.gasGiants: print("\(planet) is a gas giant")
    default:               print("I don't know")
}