function cdf( x, x0, gamma ) {
	if (
		isnan( x ) ||
		isnan( gamma ) ||
		isnan( x0 ) ||
		gamma <= 0.0
	) {
		return NaN;
	}
	return ( ONE_OVER_PI * atan2( x-x0, gamma ) ) + 0.5;
}
Evaluates the cumulative distribution function (CDF) for a Cauchy distribution with location parameter `x0` and scale parameter `gamma` at a value `x`.

@param {number} x - input value
@param {number} x0 - location parameter
@param {PositiveNumber} gamma - scale parameter
@returns {Probability} evaluated CDF

@example
var y = cdf( 4.0, 0.0, 2.0 );
// returns ~0.852

@example
var y = cdf( 1.0, 0.0, 2.0 );
// returns ~0.648

@example
var y = cdf( 1.0, 3.0, 2.0 );
// returns 0.25

@example
var y = cdf( NaN, 0.0, 2.0 );
// returns NaN

@example
var y = cdf( 1.0, 2.0, NaN );
// returns NaN

@example
var y = cdf( 1.0, NaN, 3.0 );
// returns NaN