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

	/**
	* Evaluates the quantile function for a lognormal 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 exp( mu + (sigma * qnormal( p, 0.0, 1.0 )) );
	}
}
Returns a function for evaluating the quantile function for a lognormal distribution with location parameter `mu` and scale parameter `sigma`.

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

@example
var quantile = factory( 4.0, 2.0 );
var y = quantile( 0.5 );
// returns ~54.598

y = quantile( 0.8 );
// returns ~293.901