function kurtosis( r, p ) {
	if (
		isnan( r ) ||
		isnan( p ) ||
		r <= 0.0 ||
		p < 0.0 ||
		p > 1.0
	) {
		return NaN;
	}
	return ( 6.0/r ) + ( ( p*p ) / ( ( 1.0-p ) * r ) );
}
Returns the excess kurtosis of a negative binomial distribution.

@param {PositiveNumber} r - number of failures until experiment is stopped
@param {Probability} p - success probability
@returns {NonNegativeNumber} excess kurtosis

@example
var v = kurtosis( 100, 0.2 );
// returns ~0.061

@example
var v = kurtosis( 20, 0.5 );
// returns ~0.325

@example
var v = kurtosis( 10.3, 0.8 );
// returns ~0.893

@example
var v = kurtosis( -2, 0.5 );
// returns NaN

@example
var v = kurtosis( 20, 1.1 );
// returns NaN

@example
var v = kurtosis( 20, NaN );
// returns NaN

@example
var v = kurtosis( NaN, 0.5 );
// returns NaN