function stdev( alpha, beta ) {
	var bm1;
	if (
		isnan( alpha ) ||
		alpha <= 0.0 ||
		isnan( beta ) ||
		beta <= 2.0
	) {
		return NaN;
	}
	bm1 = beta - 1.0;
	return sqrt( ( alpha * ( alpha + bm1 ) ) / ( ( bm1-1.0 ) * bm1*bm1 ) );
}
Returns the standard deviation of a beta prime distribution.

@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {PositiveNumber} standard deviation

@example
var v = stdev( 1.0, 3.0 );
// returns ~0.866

@example
var v = stdev( 4.0, 12.0 );
// returns ~0.223

@example
var v = stdev( 8.0, 2.5 );
// returns ~8.219

@example
var v = stdev( 8.0, 2.0 );
// returns NaN

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

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

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

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