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 logcdf;

	/**
	* Evaluates the logarithm of the cumulative distribution function (CDF) for a gamma distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logCDF
	*
	* @example
	* var y = logcdf( 2.0 );
	* // returns <number>
	*/
	function logcdf( x ) {
		if ( x <= 0.0 ) {
			return NINF;
		}
		if ( x === PINF ) {
			return 0.0;
		}
		return ln( gammainc( x * beta, alpha ) );
	}
}
Returns a function for evaluating the logarithm of 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} logCDF

@example
var logcdf = factory( 3.0, 1.5 );

var y = logcdf( 1.0 );
// returns ~-1.655

y = logcdf( 4.0 );
// returns ~-0.06