function kurtosis( p ) {
	var pq;
	if (
		isnan( p ) ||
		p < 0.0 ||
		p > 1.0
	) {
		return NaN;
	}
	pq = p * ( 1.0 - p );
	return ( 1.0/pq ) - 6.0;
}
Returns the excess kurtosis of a Bernoulli distribution.

@param {Probability} p - success probability
@returns {PositiveNumber} excess kurtosis

@example
var v = kurtosis( 0.1 );
// returns ~5.111

@example
var v = kurtosis( 0.5 );
// returns -2.0

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

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