Currently:
if (ve instanceof NumberEval) {
	NumberEval ne = (NumberEval) ve;
	temp.add(ne.getNumberValue());
	return;
}

Instead, it should be:
if (ve instanceof NumericValueEval) {
	NumericValueEval ne = (NumericValueEval) ve;
	temp.add(ne.getNumberValue());
	return;
}

Same is true for StringEval vs. StringValueEval.

While the current approach technically works, it provides no ability to provide custom implementations of return values from user defined functions.