@swift-4.0.3
protocol NonEmptySequence: Sequence { var first: Element { get } } struct NonEmptyArray<Element>: NonEmptySequence { private var array: [Element] init(_ array: [Element]) { precondition(!array.isEmpty) self.array = array } var first: Element { return array[0] } func makeIterator() -> Array<Element>.Iterator { return array.makeIterator() } }