#!/usr/local/bin/swift
enum NestedArray<Element> {
    case V(Element)
    case A([NestedArray])
}
var na:NestedArray = .A([
    .V(0), .V(1), .A([.V(2), .V(3)])
])
extension NestedArray {
    var value:Element? {
        switch self {
        case let .V(v):
            return v
        default:
            return nil
        }
    }
    var array:[NestedArray]? {
        switch self {
        case let .A(a):
            return a
        default:
            return nil
        }
    }
}
extension NestedArray {
    subscript(i:Int)->NestedArray? {
        get {
        switch self {
        case let .A(a):
            return a[i]
        default:
            return nil
        }
        }
        set {
            switch self {
            case var .A(a):
                a[i] = newValue!
                self = .A(a)
            default:
                return
            }
        }
    }
}

print(na[0]?.value)