function gumbel( rand, mu, beta ) {
	return mu - ( beta*ln( -ln( rand() ) ) );
}
Returns pseudorandom number drawn from a Gumbel distribution.

@private
@param {PRNG} rand - PRNG for generating uniformly distributed numbers
@param {number} mu - mean
@param {PositiveNumber} beta - scale parameter
@returns {number} pseudorandom number