function mean( a, b ) {
	if (
		isnan( a ) ||
		a <= 0.0 ||
		isnan( b ) ||
		b <= 0.0
	) {
		return NaN;
	}
	return b * beta( 1.0 + ( 1.0/a ), b );
}
Returns the expected value of a Kumaraswamy's double bounded distribution.

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

@example
var v = mean( 1.5, 1.5 );
// returns ~0.512

@example
var v = mean( 4.0, 12.0 );
// returns ~0.481

@example
var v = mean( 12.0, 2.0 );
// returns ~0.886

@example
var v = mean( 1.5, -0.1 );
// returns NaN

@example
var v = mean( -0.1, 1.5 );
// returns NaN

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

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