function ibetaDerivative( x, a, b ) {
	var f1;
	var y;
	if ( x === 1.0 ) {
		return 0.0;
	}
	// Regular cases:
	f1 = ibetaPowerTerms( a, b, x, 1.0 - x, true );
	y = ( 1.0 - x ) * x;
	f1 /= y;
	return f1;
}
Computes the partial derivative with respect to x of the incomplete beta function.

@private
@param {Probability} x - input value (0 < x <= 1)
@param {PositiveNumber} a - first parameter
@param {PositiveNumber} b - second parameter (must be greater than 1)
@returns {number} value of the partial derivative