How to compare attribute of current row to attribute of previous row in Drools

Let's say I've got a simple pojo

    public class foo {
      private String id;
      private Date asOfDate;
      private String product;

      ...getters and setters...
}

I want to feed an bunch of foos into a rule, sort them based on the date, and then compare the current product to the previous product. Out of that compare, I want to create a new fact with an additional attribute called ProductChange. If the current product = the previous product, productChange will be set to "no change". If the current product <> the previous product, set it to "change".

Here's some sample sorted data:

| id |  asOfDate  | product |
+----+------------+---------+
|  1 | 2017-01-01 | A       |
|  1 | 2017-02-01 | A       |
|  1 | 2017-03-01 | B       |
|  1 | 2017-04-01 | C       |
+----+------------+---------+

And here's what the new facts would look like:

+----+------------+---------+---------------+
| id |  asOfDate  | product | productChange |
+----+------------+---------+---------------+
|  1 | 2017-01-01 | A       | No Change     |
|  1 | 2017-02-01 | A       | No change     |
|  1 | 2017-03-01 | B       | change        |
|  1 | 2017-04-01 | C       | change        |
+----+------------+---------+---------------+

Here's what I've got so far in the rule:

rule "compare"
    when 
      $foo : foo ($id : id, $asOfDate : asOfDate, $product : product)
      not foo (asOfDate < $asOfDate)
     then
        System.out.println("id: " + $id + " date: " + $asOfDate + "product: " + $product);
     end

This gets the set sorted correctly, but I have no idea how to approach looking at a previous row.