function f( rchisq, d1, d2 ) {
	var x1 = rchisq( d1 ) / d1;
	var x2 = rchisq( d2 ) / d2;
	return x1 / x2;
}
Returns a pseudorandom number from an F distribution with parameters `d1` and `d2`.

@private
@param {PRNG} rchisq - PRNG for chi-square distributed numbers
@param {PositiveNumber} d1 - degrees of freedom
@param {PositiveNumber} d2 - degrees of freedom
@returns {NonNegativeNumber} pseudorandom number