[rules-users] not exists / forall(not) logically equivalent, behave differently

[tim tim]

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
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