function factory( mu, b ) {
	if ( isnan( mu ) || isnan( b ) || b <= 0.0 ) {
		return constantFunction( NaN );
	}
	return logcdf;

	/**
	* Evaluates the logarithm of the cumulative distribution function (CDF) for a Laplace distribution.
	*
	* @private
	* @param {number} x - input value
	* @returns {number} evaluated logCDF
	*
	* @example
	* var y = logcdf( 2.0 );
	* // returns <number>
	*/
	function logcdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		z = ( x - mu ) / b;
		if ( x < mu ) {
			return LNHALF + z;
		}
		return LNHALF + log1p( -expm1( -z ) );
	}
}
Returns a function for evaluating the logarithm of the cumulative distribution function (CDF) for a Laplace distribution with location parameter `mu` and scale parameter `b`.

@param {number} mu - location parameter
@param {PositiveNumber} b - scale parameter
@returns {Function} logCDF

@example
var logcdf = factory( 3.0, 1.5 );

var y = logcdf( 1.0 );
// returns ~-2.026

y = logcdf( 4.0 );
// returns ~-0.297