Linked List in Swift

## List.swift
/// A list is either empty or it is composed of a first element (head)
/// and a tail, which is a list itself.
///
/// See http://www.enekoalonso.com/projects/99-swift-problems/#linked-lists
class List<T> {
    var value: T
    var nextItem: List<T>?

    convenience init?(_ values: T...) {
        self.init(Array(values))
    }

    init?(_ values: [T]) {
        guard let first = values.first else {
            return nil
        }
        value = first
        nextItem = List(Array(values.suffix(from: 1)))
    }
}