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 ) );
	}
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>