function negativeBinomial2( r, p ) {
		if (
			isnan( r ) ||
			isnan( p ) ||
			p <= 0.0 ||
			p >= 1.0
		) {
			return NaN;
		}
		return rpois( rgamma( r, p/(1-p) ) );
	}
Returns a pseudorandom number drawn from a negative binomial distribution with parameters `r` and `p`.

@private
@param {PositiveNumber} r - number of successes until experiment is stopped
@param {number} p - success probability
@returns {NonNegativeInteger} pseudorandom number

@example
var v = negativeBinomial2( 10, 0.5 );
// returns <number>