function pdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		z = ( x - mu ) / b;
		return 0.5 * exp( -abs( z ) ) / b;
	}
Evaluates the probability density function (PDF) for a Laplace distribution.

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

var y = pdf( -3.14 );
// returns <number>