function kurtosis( mu, s ) {
	if (
		isnan( mu ) ||
		isnan( s ) ||
		s <= 0.0
	) {
		return NaN;
	}
	return COSINE_EXCESS_KURTOSIS;
}
Returns the excess kurtosis for a raised cosine distribution with location `mu` and scale `s`.

@param {number} mu - location parameter
@param {PositiveNumber} s - scale parameter
@returns {number} excess kurtosis

@example
var y = kurtosis( 0.0, 1.0 );
// returns ~-0.594

@example
var y = kurtosis( 5.0, 2.0 );
// returns ~-0.594

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

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

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