docs: function(size, prob) {
           return {
              d: function(x, logp) { return dnbinom(size, prob, logp)(x); },
              p: function(q, lowerTail, logp) {
                 return pnbinom(size, prob, lowerTail, logp)(q);
              },
              q: function(p, lowerTail, logp) {
                 return qnbinom(size, prob, lowerTail, logp)(p);
              },
              r: function() { return rnbinom(size, prob)(); }
           };
        }