indirect enum IntList {
    case empty
    case cons(Int, IntList)
}

extension IntList {
    static func append(_ xs: IntList, _ ys: IntList) -> IntList{
        switch xs {
            case .empty: return ys;
            case let .cons(head, tail): return .cons(head, append(tail, ys));
        }
    }
    func appending(_ xs: IntList) -> IntList {
        return IntList.append(self, xs);
    }
}

func print(list: IntList) {
    func formatter(_ list: IntList) -> String {
        switch list {
            case .empty:
            return ""
            case let .cons(head, tail):
            let separator: String
            if case .empty = tail {
                separator = ""
            } else {
                separator = ", "
            }
            return String(head) + separator + formatter(tail)
        }
    }

    print("[" + formatter(list) + "]");
}

let xList: IntList = .cons(2, .cons(1, .empty))
let yList: IntList = .cons(4, .cons(5, .empty))
let zList          = xList.appending(yList)

print(list: xList);
print(list: yList);
print(list: zList);