@Override
    protected void onSerialize(Serializer buf) {
        super.onSerialize(buf);
        int numArgs = args.size();
        buf.putInt(null, numArgs);
        for (ExpressionNode node : args) {
            serializeOptional(buf, node); // TODO: Not optional.
        }
    }
<p>Perform the appropriate calculation of the arguments into a result node.</p>

@param args   A list of operands.
@param result Place to put the result.
@return True if successful, false if not.
searchlib/src/main/java/com/yahoo/searchlib/expression/MultiArgFunctionNode.java