function cdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		z = ( x - mu ) / b;
		if ( x < mu ) {
			return 0.5 * exp( z );
		}
		return 1.0 - (0.5 * exp( -z ));
	}
Evaluates the cumulative distribution function (CDF) for a Laplace distribution.

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

@example
var y = cdf( 2.0 );
// returns <number>