@Override
    public AggregationRefNode clone() {
        AggregationRefNode obj = (AggregationRefNode)super.clone();
        obj.index = this.index;
        obj.result = this.result.clone();
        return obj;
    }
This node holds the index of an ExpressionNode in an external array, and is used as a proxy in the back-end to allow
aggregators to be used in expressions.

@author baldersheim
searchlib/src/main/java/com/yahoo/searchlib/expression/AggregationRefNode.java