public struct Pair<T>: Collection {
  var _value: (T, T)

  public init(_ first: T, _ second: T) { _value = (first, second) }
  public init(_ value: (T,T))          { _value = value }

  public var count: Index.Stride { return 2 }

  public typealias Index = Int
  public var startIndex: Index { return 0     }
  public var endIndex:   Index { return count }

  public func index(after i: Int) -> Int {
    guard i < endIndex else { preconditionFailure("Cannot increment endIndez") }
    return i + 1
  }

  public subscript(index: Index) -> T {
    get {
      switch index {
        case 0:  return _value.0
        case 1:  return _value.1
        default: fatalError("Invalid index")
      }
    }
    set {
      switch index {
        case 0:  _value.0 = newValue
        case 1:  _value.1 = newValue
        default: fatalError("Invalid index")
      }
    }
  }
}