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

	/**
	* Evaluates the quantile function for a logistic 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 + (s * ln( p / ( 1.0 - p ) ));
	}
}
Returns a function for evaluating the quantile function for a logistic distribution.

@param {number} mu - location parameter
@param {NonNegativeNumber} s - scale parameter
@returns {Function} quantile function

@example
var quantile = factory( 10.0, 2.0 );
var y = quantile( 0.5 );
// returns 10.0

y = quantile( 0.8 );
// returns ~12.773