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

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

@example
var v = mode( 1.5, 1.5 );
// returns ~0.543

@example
var v = mode( 4.0, 12.0 );
// returns ~0.503

@example
var v = mode( 12.0, 2.0 );
// returns ~0.94

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

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

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

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

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