@Override
    public boolean equalsExpression(ExpressionNode obj) {
        AggregationRefNode rhs = (AggregationRefNode)obj;
        if (index != rhs.index) {
            return false;
        }
        if (!equals(result, rhs.result)) {
            return false;
        }
        return true;
    }
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