This function computes the probability density of the Slash distribution given parameters (\(\xi\) and \(\alpha\)) provided by parsla
. The probability density function is
$$f(x) = \frac{\phi(0) - \phi(y)}{y^2} \mbox{,}$$
where \(f(x)\) is the probability density for quantile \(x\), \(y = (x - \xi)/\alpha\), \(\xi\) is a location parameter, and \(\alpha\) is a scale parameter. The function \(\phi(y)\) is the probability density function of the Standard Normal distribution.