Representing Peano integers using only Swift Optional

Peano.swift

func toPeano(_ n: Int) -> Any {
    precondition(n >= 0)
    if n == 0 {
        return nil as Any?
    } else {
        return toPeano(n - 1) as Any?
    }
}

protocol OptionalProtocol {
    var isNil: Bool { get }
    var unwrapped: Any { get }
}

extension Optional: OptionalProtocol {
    var isNil: Bool {
        return self == nil
    }
    var unwrapped: Any {
        return self!
    }
}

func fromPeano(_ n: Any) -> Int {
    let n = n as! OptionalProtocol
    if n.isNil {
        return 0
    }
    return fromPeano(n.unwrapped) + 1
}

let p = toPeano(5)
fromPeano(p)

OptionalProtocol is necessary because n as! Any? wraps n in an optional instead of casting it