function ibetaAStep( a, b, x, y, k, normalized, out ) {
	var prefix;
	var term;
	var sum;
	var i;

	prefix = ibetaPowerTerms( a, b, x, y, normalized );
	if ( out ) {
		out[ 1 ] = prefix;
	}
	prefix /= a;
	if ( prefix === 0.0 ) {
		return prefix;
	}
	sum = 1.0;
	term = 1.0;

	// Series summation from 0 to k-1:
	for ( i = 0; i < k-1; ++i ) {
		term *= (a+b+i) * x / (a+i+1.0);
		sum += term;
	}
	prefix *= sum;
	return prefix;
}
Computes the difference between `ibeta(a,b,x)` and `ibeta(a+k,b,x)`.

@private
@param {NonNegativeNumber} a - function parameter
@param {NonNegativeNumber} b - function parameter
@param {Probability} x - function parameter
@param {Probability} y - probability equal to `1-x`
@param {NonNegativeInteger} k - function input
@param {boolean} normalized - boolean indicating whether to evaluate the power terms of the regularized or non-regularized incomplete beta function
@param {(Array|TypedArray|Object)} out - output array holding the derivative as the second element
@returns {number} difference between ibeta(a,b,x) and ibeta(a+k,b,x)