Computes the significance of `x' in a Gaussian distribution with mean=`mean' (default 0.0) and standard deviation=`sd' (default 1.0); that is, it returns the area which farther from the mean than `x' is. The null hypothesis is roughly that `x' is zero; you must specify your alternative hypothesis (H1) via the `tails' parameter, which must be :both, :positive or :negative. The first corresponds to a two-tailed test: H1 is that `x' is not zero, but you are not specifying a direction. If the parameter is :positive, H1 is that `x' is positive, and similarly for :negative.