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

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

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

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

@example
var y = mode( NaN, 1.0 );
// returns NaN

@example
var y = mode( 0.0, NaN );
// returns NaN

@example
var y = mode( 0.0, 0.0 );
// returns NaN