extension Array {

    func first() -> Element? {
        if isEmpty {
            return nil
        }
        return self[0]
    }

    func last() -> Element? {
        if isEmpty {
            return nil
        }
        let index = count - 1
        return self[index]
    }

    func head() -> Element? {
        return first()
    }

    func tail() -> [Element]? {
        if isEmpty || count == 1 {
            return nil
        }
        let range: Range<Int> = Range(start: 1, end: count)
        let slice = self[range]
        return Array(slice)
    }

}