function median( a, b ) {
	if (
		isnan( a ) ||
		a <= 0.0 ||
		isnan( b ) ||
		b <= 0.0
	) {
		return NaN;
	}
	return pow( 1.0 - pow( 2.0, -1.0/b ), 1.0/a );
}
Returns the median of a Kumaraswamy's double bounded distribution.

@param {PositiveNumber} a - first shape parameter
@param {PositiveNumber} b - second shape parameter
@returns {PositiveNumber} median

@example
var v = median( 0.5, 1.0 );
// returns 0.25

@example
var v = median( 4.0, 12.0 );
// returns ~0.487

@example
var v = median( 12.0, 2.0 );
// returns ~0.903

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

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

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

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