function logcdf( x, mu, b ) {
	return addon( x, mu, b );
}
Evaluates the logarithm of the cumulative distribution function (CDF) for a Laplace distribution with location parameter `mu` and scale parameter `b` at value `x`.

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

@example
var y = logcdf( 2.0, 0.0, 1.0 );
// returns ~-0.07

@example
var y = logcdf( 5.0, 10.0, 3.0 );
// returns ~-2.36

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

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

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

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