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

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

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

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

var y = logcdf( 10.0 );
// returns ~-0.693

y = logcdf( 12.0 );
// returns ~-0.288