code: function rnbinom(size, prob) {
        var rg;
  
        if (size <= 0 || prob <= 0 || prob > 1) {
           return function() { return NaN; };
        }
        if (prob === 1) { return function() { return 0; }; }
        rg = rgamma(size, (1 - prob) / prob);
        return function() { return rpois(rg())(); };
     }
docs: Returns a random variate from the negative binomial distribution.
  
  `size` must be strictly positive, and `prob` must be in `(0, 1]`.
  
  @fullName rnbinom(size, prob)()
  @memberof nbinom