function kurtosis( alpha, beta ) {
	if (
		isnan( alpha ) ||
		isnan( beta ) ||
		alpha <= 0.0 ||
		beta <= 0.0
	) {
		return NaN;
	}
	return 6.0 / alpha;
}
Returns the excess kurtosis of a gamma distribution.

@param {PositiveNumber} alpha - shape parameter
@param {PositiveNumber} beta - rate parameter
@returns {PositiveNumber} excess kurtosis

@example
var v = kurtosis( 1.0, 1.0 );
// returns 6.0

@example
var v = kurtosis( 4.0, 12.0 );
// returns 1.5

@example
var v = kurtosis( 8.0, 2.0 );
// returns 0.75

@example
var v = kurtosis( 1.0, -0.1 );
// returns NaN

@example
var v = kurtosis( -0.1, 1.0 );
// returns NaN

@example
var v = kurtosis( 2.0, NaN );
// returns NaN

@example
var v = kurtosis( NaN, 2.0 );
// returns NaN