function factory( x0, gamma ) {
	if (
		isnan( gamma ) ||
		isnan( x0 ) ||
		gamma <= 0.0
	) {
		return constantFunction( NaN );
	}
	return cdf;

	/**
	* Evaluates the cumulative distribution function (CDF) for a Cauchy distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {Probability} evaluated CDF
	*
	* @example
	* var y = cdf( 2.0 );
	* // returns <number>
	*/
	function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return ( ONE_OVER_PI * atan2( x-x0, gamma ) ) + 0.5;
	}
}
Returns a function for evaluating the cumulative distribution function (CDF) for a Cauchy distribution with location parameter `x0` and scale parameter `gamma`.

@param {number} x0 - location parameter
@param {PositiveNumber} gamma - scale parameter
@returns {Function} CDF

@example
var cdf = factory( 10.0, 2.0 );

var y = cdf( 10.0 );
// returns 0.5

y = cdf( 12.0 );
// returns 0.75