function logcdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		z = ( x - mu ) / beta;
		return -exp( -z );
	}
Evaluates the logarithm of the cumulative distribution function (CDF) for a Gumbel distribution.

@private
@param {number} x - input value
@returns {number} evaluated logarithm of CDF

@example
var y = logcdf( 2.0 );
// returns <number>