function variance( alpha, beta ) {
	return addon( alpha, beta );
}
Returns the variance of a beta distribution.

@private
@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {PositiveNumber} variance

@example
var v = variance( 1.0, 1.0 );
// returns ~0.083

@example
var v = variance( 4.0, 12.0 );
// returns ~0.011

@example
var v = variance( 8.0, 2.0 );
// returns ~0.015

@example
var v = variance( 1.0, -0.1 );
// returns NaN

@example
var v = variance( -0.1, 1.0 );
// returns NaN

@example
var v = variance( 2.0, NaN );
// returns NaN

@example
var v = variance( NaN, 2.0 );
// returns NaN