function sample( rand, a, b ) {
	var u = rand();
	return pow( 1.0 - pow( 1.0-u, 1.0/b ), 1.0/a );
}
Returns a random number drawn from Kumaraswamy's double bounded distribution.

@private
@param {PRNG} rand - pseudorandom number generator for uniformly distributed numbers
@param {PositiveNumber} a - first shape parameter
@param {PositiveNumber} b - second shape parameter
@returns {Probability} pseudorandom number