function cdf( x ) {
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x < 0.0 ) {
			return 0.0;
		}
		return gamma( x*x );
	}
Evaluates the cumulative distribution function (CDF) for a chi distribution with degrees of freedom `k`.

@private
@param {number} x - input value
@returns {Probability} evaluated CDF