function weibull( rand, k, lambda ) {
	return lambda * pow( -ln( 1.0-rand() ), 1.0/k );
}
Returns a pseudorandom number drawn from a Weibull distribution.

@private
@param {PRNG} rand - PRNG for generating uniformly distributed numbers
@param {PositiveNumber} k - shape parameter
@param {PositiveNumber} lambda - scale parameter
@returns {NonNegativeNumber} pseudorandom number