Accessing both the new and old value of an attribute

[eithe]

Hi, I'm new to NRules so bear with me, but I'm somehow missing how to access the previous value of an attribute. Say I want to define a rule that only should fire if Status specifically has changed from "Processing" to "Done".

public class MyObject
{
    public int Id { get; set; }
    public string Status { get; set; }
}

public class ObjectNameChangedRule : Rule
{
    public override void Define()
    {
        MyObject myObj = null;
        When()
            .Match(() => myObj , o => myObj.Status == "Done")
           // match oldObject.Status to "Processing" ?
    }
}