function gamma( randu, randn, beta, d, c ) {
	var flg;
	var x2;
	var v0;
	var v1;
	var x;
	var u;
	var v;

	flg = true;
	while ( flg ) {
		do {
			x = randn();
			v = 1.0 + (c*x);
		} while ( v <= 0.0 );
		v *= v * v;
		x2 = x * x;
		v0 = 1.0 - (0.331*x2*x2);
		v1 = (0.5*x2) + (d*( 1.0-v+ln(v) ));
		u = randu();
		if ( u < v0 || ln( u ) < v1 ) {
			flg = false;
		}
	}
	return (1.0/beta) * d * v;
}
Returns a pseudorandom number drawn from a gamma distribution.

@private
@param {PRNG} randu - PRNG for uniformly distributed numbers
@param {PRNG} randn - PRNG for standard normally distributed numbers
@param {PositiveNumber} beta - rate parameter
@param {PositiveNumber} d - `alpha + 2/3` or `alpha - 1/3`
@param {PositiveNumber} c - `1.0 / sqrt( 9.0*d )`
@returns {PositiveNumber} pseudorandom number