function variance( mu, sigma ) {
	if (
		isnan( mu ) ||
		isnan( sigma ) ||
		sigma <= 0.0
	) {
		return NaN;
	}
	return sigma*sigma;
}
Returns the variance for a normal distribution with mean `mu` and standard deviation `sigma`.

@param {number} mu - mean
@param {PositiveNumber} sigma - standard deviation
@returns {PositiveNumber} variance

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

@example
var y = variance( 5.0, 3.0 );
// returns 9.0

@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