push/pop state property wrapper

Stack.swift

@propertyWrapper
struct Stack<Value> {
    private var history: [Value]

    init(wrappedValue initialValue: Value) {
        self.history = [initialValue]
    }

    var wrappedValue: Value {
        get { history[history.endIndex - 1] }
        _modify { yield &history[history.endIndex - 1] }
        set { history[history.endIndex - 1] = newValue }
    }

    var projectedValue: Self {
        return self
    }

    mutating func push() {
        history.append(wrappedValue)
    }
    
    mutating func pop() throws {
        guard history.count > 1 else { throw UnderflowError() }
        history.removeLast()
    }
}
struct UnderflowError: Error { }

fyi Underflow error is not nested bc of compiler bug

similar: https://gist.github.com/JadenGeller/4344fc84e8073f7a63985a2c2a4a7e4e