function normal2( mu, sigma ) {
		if (
			isnan( mu ) ||
			isnan( sigma ) ||
			sigma <= 0.0
		) {
			return NaN;
		}
		return normal0( rnorm, mu, sigma );
	}
Returns a normally distributed pseudorandom number with mean `mu` and standard deviation `sigma`.

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

@example
var v = normal2( 0.0, 1.0 );
// returns <number>

@example
var v = normal2( 0.0, -1.0 );
// returns NaN