optionSet2.swift

enum ShippingOperation {
  case nextDay
  case secondDay
  case priority
  case standard
}

typealias ShippingOperationSet = Set<ShippingOperation>

extension ShippingOperationSet {
  static var express: ShippingOperationSet { [.nextDay, .secondDay]}
  static var all: ShippingOperationSet { return ShippingOperationSet.express.union([.priority, .standard]) }
}