function variance( mu, s ) {
	return addon( mu, s );
}
Returns the variance for a raised cosine distribution with location `mu` and scale `s`.

@private
@param {number} mu - location parameter
@param {PositiveNumber} s - scale parameter
@returns {number} variance

@example
var y = variance( 0.0, 1.0 );
// returns ~0.131

@example
var y = variance( 5.0, 2.0 );
// returns ~0.523

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

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

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