function logcdf( x, mu, beta ) {
	var z;
	if (
		isnan( x ) ||
		isnan( mu ) ||
		isnan( beta ) ||
		beta <= 0.0
	) {
		return NaN;
	}
	z = ( x - mu ) / beta;
	return -exp( -z );
}
Evaluates the logarithm of the cumulative distribution function (CDF) for a Gumbel distribution with location parameter `mu` and scale parameter `beta` at a value `x`.

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

@example
var y = logcdf( 10.0, 0.0, 3.0 );
// returns ~-0.036

@example
var y = logcdf( -2.0, 0.0, 3.0 );
// returns ~-1.948

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

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

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

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

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