This rule

rule "test"
when
    $l : ArrayList() from collect (MyClass (attribute == false));
then
    for(Object o : new ArrayList( $l )) {
        MyClass o2 = (MyClass) o;
        modify(o2) { setAttribute(true) }
    }
end

does not compile:  The method setAttribute(boolean) is undefined for the type Object

This, however, works:
   modify( (MyClass)o) { setAttribute(true) }

Explanation?
-W