function beta2( alpha, beta ) {
		if (
			isnan( alpha ) ||
			isnan( beta ) ||
			alpha <= 0.0 ||
			beta <= 0.0
		) {
			return NaN;
		}
		return beta0( rand, rnorm, alpha, beta );
	}
Returns a random number drawn from a beta distribution.

@private
@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {Probability} pseudorandom number

@example
var v = beta2( 2.0, 3.0 );
// returns <number>