function logpdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x === NINF ) {
			return 0.0;
		}
		z = ( x - mu ) / beta;
		return -z - exp( -z ) - lbeta;
	}
Evaluates the logarithm of the probability density function (PDF) for a Gumbel distribution.

@private
@param {number} x - input value
@returns {number} evaluated logPDF

@example
var y = logpdf( 2.3 );
// returns <number>