code: func Exponential(factor time.Duration, base float64) Algorithm {
  	return func(attempt uint) time.Duration {
  		return (factor * time.Duration(math.Pow(base, float64(attempt))))
  	}
  }
docs: // Exponential creates a Algorithm that multiplies the factor duration by
  // an exponentially increasing factor for each attempt, where the factor is
  // calculated as the given base raised to the attempt number.