function factory( a, b ) {
	if (
		isnan( a ) ||
		isnan( b ) ||
		a <= 0.0 ||
		b <= 0.0
	) {
		return constantFunction( NaN );
	}
	return quantile;

	/**
	* Evaluates the quantile function for a Kumaraswamy's double bounded distribution.
	*
	* @private
	* @param {Probability} p - input probability
	* @returns {number} evaluated quantile function
	*
	* @example
	* var y = quantile( 2.0 );
	* // returns <number>
	*/
	function quantile( p ) {
		if ( isnan( p ) || p < 0.0 || p > 1.0 ) {
			return NaN;
		}
		return pow( 1.0 - pow( 1.0-p, 1.0/b ), 1.0/a );
	}
}
Returns a function for evaluating the quantile function for a Kumaraswamy's double bounded distribution with first shape parameter `a` and second shape parameter `b`.

@param {PositiveNumber} a - first shape parameter
@param {PositiveNumber} b - second shape parameter
@returns {Function} quantile function

@example
var quantile = factory( 0.5, 0.5 );

var y = quantile( 0.8 );
// returns ~0.922

y = quantile( 0.3 );
// returns ~0.26