docs: 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())(); };
     }