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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def normal_cdf(x, mu=0, sigma=1):
 """Cumulative Normal Distribution Function. :param x: scalar or array of real numbers. :type x: numpy.ndarray, float :param mu: Mean value. Default 0. :type mu: float, numpy.ndarray :param sigma: Standard deviation. Default 1. :type sigma: float :returns: An approximation of the cdf of the normal. :rtype: numpy.ndarray Note: CDF of the normal distribution is defined as \frac12 [1 + erf(\frac{x - \mu}{\sigma \sqrt{2}})], x \in \R Source: http://en.wikipedia.org/wiki/Normal_distribution """

    arg = (x - mu) / (sigma * numpy.sqrt(2))
    res = (1 + erf(arg)) / 2
    return res