function factory( r, p ) {
	if (
		isnan( r ) ||
		isnan( p ) ||
		r <= 0.0 ||
		p < 0.0 ||
		p > 1.0
	) {
		return constantFunction( NaN );
	}
	return cdf;

	/**
	* Evaluates the cumulative distribution function (CDF) for a negative binomial distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated CDF
	*/
	function cdf( x ) {
		var xint;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 ) {
			return 0.0;
		}
		if ( x === PINF ) {
			return 1.0;
		}
		// Ensure left-continuity:
		xint = floor( x + 1e-7 );
		return betainc( p, r, xint + 1.0 );
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for a negative binomial distribution with number of successes until experiment is stopped `r` and success probability `p`.

@param {PositiveNumber} r - number of successes until experiment is stopped
@param {Probability} p - success probability
@returns {Function} CDF

@example
var cdf = factory( 10, 0.5 );
var y = cdf( 3.0 );
// returns ~0.046

y = cdf( 11.0 );
// returns ~0.668