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

	/**
	* Evaluates the quantile function for a Gumbel distribution.
	*
	* @private
	* @param {Probability} p - input value
	* @returns {number} evaluated quantile function
	*
	* @example
	* var y = quantile( 0.3 );
	* // returns <number>
	*/
	function quantile( p ) {
		if ( isnan( p ) || p < 0.0 || p > 1.0 ) {
			return NaN;
		}
		return mu - (beta * ln( -ln( p ) ));
	}
}
Returns a function for evaluating the quantile function for a Gumbel distribution with location parameter `mu` and scale parameter `beta`.

@param {number} mu - location parameter
@param {PositiveNumber} beta - scale parameter
@returns {Function} quantile function

@example
var quantile = factory( 0.0, 1.0 );
var y = quantile( 0.2 );
// returns ~-0.476