function sample( rgamma, alpha, beta ) {
	return rgamma( alpha, 1.0 ) / rgamma( beta, 1.0 );
}
Returns a random number drawn from a beta prime distribution.

@private
@param {PRNG} rgamma - pseudorandom number generator for gamma distributed numbers
@param {PositiveNumber} alpha - first shape parameter
@param {PositiveNumber} beta - second shape parameter
@returns {PositiveNumber} pseudorandom number