def nd_denormalize(x, mu, sigma):
    """Restore `x` to its oirignal mean (mu) and deviation (sigma)

    Thus undoes nd_normalize.

    :param x:
    :param mu: The mean of the (original) distribution of x vectors.
    :param sigma: The standard deviation of the (original) distribution of x vectors.
    """
    return (x * sigma) + mu