function cdf( x, k ) {
	return gammaCDF( x, k/2.0, 0.5 );
}
Evaluates the cumulative distribution function (CDF) for a chi-squared distribution with degrees of freedom `k` at a value `x`.

@param {number} x - input value
@param {NonNegativeNumber} k - degrees of freedom
@returns {Probability} evaluated CDF

@example
var y = cdf( 2.0, 3.0 );
// returns ~0.428

@example
var y = cdf( 1.0, 0.5 );
// returns ~0.846

@example
var y = cdf( -1.0, 4.0 );
// returns 0.0

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

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

@example
// Negative degrees of freedom:
var y = cdf( 2.0, -1.0 );
// returns NaN