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

	/**
	* Evaluates the probability density function (PDF) for a logistic distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated PDF
	*
	* @example
	* var y = pdf( -1.2 );
	* // returns <number>
	*/
	function pdf( x ) {
		var ez;
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x === NINF ) {
			return 0.0;
		}
		z = abs( ( x - mu ) / s );
		ez = exp( -z );
		return ez / ( s * pow( 1.0 + ez, 2.0 ) );
	}
}
Returns a function for evaluating the probability density function (PDF) for a logistic distribution.

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

@example
var pdf = factory( 10.0, 2.0 );
var y = pdf( 10.0 );
// returns 0.125

y = pdf( 5.0 );
// returns ~0.035