function kurtosis( k, lambda ) {
	var out;
	var g4;
	var g3;
	var g2;
	var g1;
	if (
		isnan( k ) ||
		isnan( lambda ) ||
		k <= 0.0 ||
		lambda <= 0.0
	) {
		return NaN;
	}
	g1 = gamma( 1.0 + (1.0/k) );
	g2 = gamma( 1.0 + (2.0/k) );
	g3 = gamma( 1.0 + (3.0/k) );
	g4 = gamma( 1.0 + (4.0/k) );
	out = (-6.0*pow(g1, 4.0)) + (12.0*g1*g1*g2) - (3.0*g2*g2)- (4.0*g1*g3) + g4;
	out /= pow( g2 - (g1*g1), 2.0 );
	return out;
}
Returns the excess kurtosis of a Weibull distribution.

@param {PositiveNumber} k - shape parameter
@param {PositiveNumber} lambda - scale parameter
@returns {number} excess kurtosis

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

@example
var v = kurtosis( 4.0, 12.0 );
// returns ~-0.252

@example
var v = kurtosis( 8.0, 2.0 );
// returns ~0.328

@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