Function Poisson-Cdf

Part of:

package cl-mathstats
( poisson-cdf < k > < x > )

Computes the cumulative distribution function for a Poisson random variable
with mean x&#39; evaluated at k.' The result is the probability that the number of
Poisson random events occurring will be between 0 and k-1 inclusive, if the
expected number is x.&#39; The argument k' should be an integer, while `x' should
be a float. The implementation follows Numerical Recipes in C, section 6.2