function risingFactorialRatio( a, b, k ) {
	var result;
	var i;
	if ( k === 0 ) {
		return 1.0;
	}
	result = 1.0;
	for ( i = 0; i < k; i++ ) {
		result *= ( a + i ) / ( b + i );
	}
	return result;
}
Computes the delta in `beta(a,b,x) = prefix + delta * beta(a+k,b,x)`.

## Notes

Specifically, the function calculates

```tex
\frac{ (a)(a+1)(a+2)...(a+k-1) }{ (b)(b+1)(b+2)...(b+k-1) }
```

The function should only called with small `k`; for large `k`, it is grossly inefficient.

@private
@param {number} a - input value
@param {number} b - input value
@param {NonNegativeInteger} k - input value
@returns {number} ratio value