[rules-users] "not exists MyObject" - is it a valid LHS condition?

Hi, I wonder if the combination of 'not' and 'exists' keywords is a valid one and therefore the construct like rule "myrule" when not exist MyObject(param matches "somepattern") then System.out.println("Object for pattern 'somepattern' doesn't exisit"); End Thanks. Vlad PS: basically I need to trigger a rule if some set of facts is NOT found and I need to trigger it only once.