Drools condition for when fact exists or does not exist - drools

Question

I'm trying to set up a drools rule that would achieve something like this:
rule "Rule 1"
salience 100
activation-group "..."
when
$fact: Fact exists, or fact does not exist
then
if $fact exists then do X. Else, do Y.
I know I can do this by having two different rules, one for when the fact exists, and one for when the fact does not exist. Is there any way I can combine these conditions into one? For the when statement, I have tried:
$fact: Fact() || not Fact()
which didn't work. Is there a correct way to write the "when" statement in drools?

Answer 1

If we revert to boolean logic, A || !A is the same as true so you can omit the condition entirely.
You're already doing this correctly. Rule 1: if X exists, then do A(). Rule 2: if X does not exist, then do B(). That is the correct way to do things:
rule "Fact exists"
when
exists(Fact())
then
doX()
end
rule "Fact does not exist"
when
not(Fact())
then
doY()
end
That's good rule design and how you should do this. I can't think of a good reason to compress it into one rule. If you can clarify a legitimate use case for why you'd want to collapse it into a single rule, it might be possible to give a better or more concrete response.
You may be able to abuse conditional and named consequences for this, though it's definitely not designed for this (nor recommended.)