[rules-users] Creating attributes of facts

Hi, I need to create properties/attributes of a fact. Like today I am checking for Color of a box. Tomorrow I want to Check Width also of a Box. Can I make this change just in the rule and attach it to some object and pass it forward through the rule chain? I do not want to add Width Attribute to Box.. as I will have to recompile the java files also in that case. I do not see this scenario in the documentaion. Where can I find better examples. Thanks, Amit