@Override
    public void onPrepare() {
        super.onPrepare();

        ResultNode result = getResult();
        if (!(result instanceof IntegerResultNode) &&
            !(result instanceof FloatResultNode) &&
            !(result instanceof StringResultNode) &&
            !(result instanceof RawResultNode))
        {
            throw new RuntimeException("Can not perform numeric function on value of type '" +
                                       getResult().getClass().getName() + "'.");
        }
    }
This is an abstract class for all functions that perform arithmetics. This node implements the necessary API for
doing arithmetic operations.

@author Ulf Lilleengen
searchlib/src/main/java/com/yahoo/searchlib/expression/NumericFunctionNode.java