function logpdf( x, mu, s ) {
	var az;
	var z;
	if (
		isnan( x ) ||
		isnan( mu ) ||
		isnan( s ) ||
		s < 0.0
	) {
		return NaN;
	}
	if ( x === NINF ) {
		return NINF;
	}
	if ( s === 0.0 ) {
		return ( x === mu ) ? PINF : NINF;
	}
	z = ( x - mu ) / s;
	az = -abs( z );
	return az - (2.0 * log1p( exp( az ) )) - ln( s );
}
Evaluates the logarithm of the probability density function (PDF) for a logistic distribution with location parameter `mu` and scale parameter `s` at a value `x`.

@param {number} x - input value
@param {number} mu - location parameter
@param {NonNegativeNumber} s - scale parameter
@returns {number} evaluated logPDF

@example
var y = logpdf( 2.0, 0.0, 1.0 );
// returns ~-2.254

@example
var y = logpdf( -1.0, 4.0, 2.0 );
// returns ~-3.351

@example
var y = logpdf( NaN, 0.0, 1.0 );
// returns NaN

@example
var y = logpdf( 0.0, NaN, 1.0 );
// returns NaN

@example
var y = logpdf( 0.0, 0.0, NaN );
// returns NaN

@example
// Negative scale parameter:
var y = logpdf( 2.0, 0.0, -1.0 );
// returns NaN

@example
var y = logpdf( 2.0, 8.0, 0.0 );
// returns -Infinity

@example
var y = logpdf( 8.0, 8.0, 0.0 );
// returns Infinity