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

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

@example
var y = variance( 0.0, 1.0 );
// returns 2.0

@example
var y = variance( 5.0, 2.0 );
// returns 8.0

@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