function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		return ( ONE_OVER_PI * atan2( x-x0, gamma ) ) + 0.5;
	}
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>