function pdf( x ) {
		var z;
		if ( isnan( x ) ) {
			return NaN;
		}
		if ( x === NINF ) {
			return 0.0;
		}
		z = ( x - mu ) / beta;
		return ( 1.0 / beta ) * exp( -z - exp( -z ) );
	}
Evaluates the probability density function (PDF) for a Gumbel distribution.

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

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