edson, thanks a lot for your answer. now i came to a new problem. i stated the rule as you said: rule "no dog without a tail" when $dog :Dog() not( Tail(dog == $dog) ) then // fire rule end but now, if i do workingMemory.insert(dog); workingMemory.insert(dog.getTail()); workingMemory.fireAllRules(); modifyRetract(dogHandle); dog.changeHashCode(); // do something that changes the hashCode of the dog modifyInsert(dogHandle); workingMemory.fireAllRules(); in the second fireAllRules() the rule fires, even though there is a tail for the dog. am i not allowed to do an opperation that changes the hashCode of the modified object between the modifyRetract() and modifyInsert() ? should i totally retract the object, and insert it again instead? thank you, tim 2008/10/6 tim tim <timbaermannextra@...> hello there, i have a rule, that should fire, when a required part of a whole is missing. lets say the whole is a dog, and the required part the tail. then the rule should fire, when there is a dog without a tail. i stated the rule in two logically equivalent ways. 1) when we have a dog, and all existing tails belong to other peoples dogs, the rule should fire: rule "no dog without a tail; forall" when $dog :Dog() forall($t : Tail() Tail(dog != $dog)) then // fire rule end 2) when we have a dog and no tail exists that belongs to this dog the rule should fire: rule "no dog without a tail; not exists" when $dog :Dog() not( exists( Tail(dog == $dog))) then // fire rule end and they really do behave similar, but not the same.. workingMemory.insert(dog); workingMemory.insert(dog.getTail()); workingMemory.fireAllRules(); // here both did not fire, but when i go on: workingMemory.modifyRetract(dogHandle) workingMemory.modifyInsert(dogHandle, dog) workingMemory.fireAllRules(); // now the not(exists(..)) version did fire, whereas the forall(..) version did not. of course both rules should not have fired, nothing changed for the dogs relationship with its tail. i am using the 5.0.0.snapshot version of drools but i can not imagine that this is an unknown bug, as this is rather basic predicate logic. what am i missing? best, tim