import Foundation

let anArray = [1, 2, 3, 4, 5]


extension Array where Element: Equatable {
    func descendingElementalPairs() -> Array<Array<Element>> {
        return self.enumerated()
            .map { pair -> Array<Array<Element>> in
                self[(pair.offset + 1)...].map {
                    [pair.element, $0]
                }
            }.reduce([[Element]]()) { $0 + $1 }
    }
}


anArray.descendingElementalPairs().forEach {
    print("\($0)")
}