This function computes the cumulative probability or nonexceedance probability of the Slash distribution given parameters (\(\xi\) and \(\alpha\)) of the distribution provided by parsla or vec2par. The cumulative distribution function is
$$F(x) = \Phi(Y) - [\phi(0) - \phi(Y)]/Y \mbox{,}$$
for \(Y \ne 0\) and
$$F(x) = 1/2 \mbox{,}$$
for \(Y = 0\), where \(F(x)\) is the nonexceedance probability for quantile \(x\), \(Y = (x - \xi)/\alpha\), \(\xi\) is a location parameter, and \(\alpha\) is a scale parameter. The function \(\Phi(Y)\) is the cumulative distribution function of the Standard Normal distribution, and \(\phi(Y)\) is the probability density function of the Standard Normal distribution.