function cdf( x, mu, b ) {
	var z;
	if (
		isnan( x ) ||
		isnan( mu ) ||
		isnan( b ) ||
		b <= 0.0
	) {
		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 with location parameter `mu` and scale parameter `b` at value `x`.

@param {number} x - input value
@param {number} mu - location parameter
@param {PositiveNumber} b - scale parameter
@returns {Probability} evaluated CDF

@example
var y = cdf( 2.0, 0.0, 1.0 );
// returns ~0.932

@example
var y = cdf( 5.0, 10.0, 3.0 );
// returns ~0.094

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

@example
var y = cdf( 2, NaN, 1.0 );
// returns NaN

@example
var y = cdf( 2.0, 0.0, NaN );
// returns NaN

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