function factory( alpha, beta ) {
	if (
		isnan( alpha ) ||
		isnan( beta ) ||
		alpha < 0.0 ||
		beta <= 0.0
	) {
		return constantFunction( NaN );
	}
	if ( alpha === 0.0 ) {
		return degenerate( 0.0 );
	}
	return cdf;

	/**
	* Evaluates the cumulative distribution function (CDF) for a gamma distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated CDF
	*
	* @example
	* var y = cdf( 2.0 );
	* // returns <number>
	*/
	function cdf( x ) {
		if ( x <= 0.0 ) {
			return 0.0;
		}
		if ( x === PINF ) {
			return 1.0;
		}
		return gammainc( x * beta, alpha );
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for a gamma distribution with shape parameter `alpha` and rate parameter `beta`.

@param {NonNegativeNumber} alpha - shape parameter
@param {PositiveNumber} beta - rate parameter
@returns {Function} CDF

@example
var cdf = factory( 0.5, 0.1 );
var y = cdf( 12.0 );
// returns ~0.879

y = cdf( 8.0 );
// returns ~0.794