how can I bind to a tuple in the closure?

mse.swift

func meanSquaredError(predictions: [Double], values: [Double]) -> Double {
    let sum = reduce(Zip2(predictions, values), 0.0) { (acc, y) in
        let diff = y.0 - y.1
        return acc + (diff * diff)
    }
    return sum / Double(predictions.count)
}

// what I want is to bind the arguments in the closure to (acc, (yHat, y))