Basic Prolog problem

From: Tjitze Rienstra (tjitze_at_gmail.com)
Date: 10/04/04 

Date: 4 Oct 2004 13:41:10 -0700

Hi

I'm working with a knowledge base for which I wrote some predicates
that assert facts which can be deducted from facts that are already
present. The details don't really matter, what I get stuck at is this:

If fact(A, B) is true then fact(B, A) is also true

I could assert fact(A, B) if fact(B, A) exists, which is what I did,
but then I thought that's not a nice solution because it really is
redundant data. I therefore created the rule

fact(A, B) :- fact(B, A).

which, for a reason that was obvious when I saw the output, doesn't
work when fact(B, A) doesn't exist.

Is there a way to implement this rule in a correct manner, without
asserting the facts? I'm sure this is basic stuff but I can't find the
solution anywhere.

Thanks in advance,

T

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