private double evaluateInternal(ValueVector x, ValueVector y, int size)
throws EvaluationException {
Accumulator acc = createAccumulator();
// error handling is as if the x is fully evaluated before y
ErrorEval firstXerr = null;
ErrorEval firstYerr = null;
boolean accumlatedSome = false;
double result = 0.0;
for (int i = 0; i < size; i++) {
ValueEval vx = x.getItem(i);