The Poisson distribution is a special case for large successes
-poisson([lambda]) = lim [sub r [rarr] [infin]] [space] negative_binomial(r, r / ([lambda] + r)))
+poisson([lambda]) = lim [sub r [rarr] [infin]] negative_binomial(r, r / ([lambda] + r)))
[discrete_quantile_warning Negative Binomial]