function invgamma2( alpha, beta ) {
		if (
			isnan( alpha ) ||
			isnan( beta ) ||
			alpha <= 0.0 ||
			beta <= 0.0
		) {
			return NaN;
		}
		return 1.0 / rgamma( alpha, beta );
	}
Returns a pseudorandom number drawn from an inverse gamma distribution.

@private
@param {PositiveNumber} alpha - shape parameter
@param {PositiveNumber} beta - scale parameter
@returns {number} pseudorandom number

@example
var v = invgamma2( 2.0, 1.0 );
// returns <number>