function logpdf( x ) {
		var az;
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x === NINF ) {
			return NINF;
		}
		z = ( x - mu ) / s;
		az = -abs( z );
		return az - (2.0 * log1p( exp( az ) )) - ls;
	}
Evaluates the logarithm of the probability density function (PDF) for a logistic distribution.

@private
@param {number} x - input value
@returns {number} evaluated logPDF

@example
var y = logpdf( -1.2 );
// returns <number>