valueWithDerivative(at:_:of:)

func valueWithDerivative<T, U, R>(at x: T, _ y: U, of f: @escaping (T, U) -> R) -> (value: R, derivative: R.TangentVector) where T : FloatingPoint, T : Differentiable, T == T.TangentVector, U : FloatingPoint, U : Differentiable, U == U.TangentVector, R : Differentiable