function logpdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		z = ( x - mu ) / b;
		return -( abs( z ) + ln( 2.0 * b ) );
	}
Evaluates the logarithm of the probability density function (PDF) for a Laplace distribution.

@private
@param {number} x - input value
@returns {number} evaluated logarithm of PDF

@example
var y = logpdf( -3.14 );
// returns <number>