function variance( alpha, beta ) {
	var apb;
	var out;
	if ( alpha <= 0.0 || beta <= 0.0 ) {
		return NaN;
	}
	apb = alpha + beta;
	out = alpha * beta;
	out /= apb * apb * ( apb+1.0 );
	return out;
}
Returns the variance of a beta distribution.

@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