ion represents
 the distribution of the number of failures before <code>r</code> successes occur.
 This is the convention adopted in e.g.
 <a href="http://mathworld.wolfram.com/NegativeBinomialDistribution.html">MathWorld</a>,
 but <em>not</em> in
 <a href="http://en.wikipedia.org/wiki/Negative_binomial_distribution">Wikipedia</a>.
 </p>
 <p>
 For a random variable <code>X</code> whose values are distributed according to this
 distribution, the probability mass function is given by<br/>
 <code>P(X = k) = C(