tive_binomial_distribution</span></code>
          represents a <a href="http://en.wikipedia.org/wiki/Negative_binomial_distribution" target="_top">negative_binomial
          distribution</a>: it is used when there are exactly two mutually exclusive
          outcomes of a <a href="http://en.wikipedia.org/wiki/Bernoulli_trial" target="_top">Bernoulli
          trial</a>: these outcomes are labelled "success" and "failure".
        </p>
<p>
          For k + r Bernoulli trials each with success fraction p, 