JESS: Backward Chaining Problem

[Robert Gaimari]

I'm having an odd problem.  Here is a run to show what's happening.

-------------------------------------------

Jess, the Java Expert System Shell
Copyright (C) 2001 E.J. Friedman Hill and the Sandia Corporation
Jess Version 6.1b3 2/28/2003
Jess>
(deftemplate foo
  (slot bar))
(deftemplate mine
  (slot me))
(do-backward-chaining foo)

(defrule get-a-foo
  ?fact <- (need-foo)
  =>
  (printout t "need a foo" crlf)
  (assert (foo (bar ber)))
  (retract ?fact))
(defrule want-a-foo
  (mine (me ?name))
  (foo (bar bie))
  =>
  (printout t ?name " has a  " ?b crlf))
;; note here that what gets asserted in get-a-foo is not
;; what want-a-foo is looking for.
(defrule dont-want-two-foos
  (explicit (foo))
  ?fact <- (need-foo)
  =>
  (printout t "don't need two" crlf)
  (retract ?fact))
;; note here that I'm not specifying the value in 'bar' for either
;; foo or need-foo in this test.  just that there is one and the other.
TRUE

Jess> (watch all)
TRUE
Jess> (reset)
 ==> Focus MAIN
 ==> f-0 (MAIN::initial-fact)
 ==> Focus MAIN
 ==> f-0 (MAIN::initial-fact)
TRUE
Jess> (assert (mine (me bob)))
 ==> f-1 (MAIN::mine (me bob))
 ==> f-2 (MAIN::need-foo (bar bie))
==> Activation: MAIN::get-a-foo :  f-2,
FALSE
Jess> (facts)
f-0   (MAIN::initial-fact)
f-1   (MAIN::mine (me bob))
f-2   (MAIN::need-foo (bar bie))
For a total of 3 facts.
Jess> (run)
FIRE 1 MAIN::get-a-foo f-2,
need a foo
 ==> f-3 (MAIN::foo (bar ber))
==> Activation: MAIN::dont-want-two-foos :  f-3, f-2,
<== Activation: MAIN::dont-want-two-foos :  f-3, f-2,
 <== f-2 (MAIN::need-foo (bar bie))
 <== Focus MAIN
1
Jess> (facts)
f-0   (MAIN::initial-fact)
f-1   (MAIN::mine (me bob))
f-3   (MAIN::foo (bar ber))
For a total of 3 facts.
;; I didn't expect want-a-foo to fire, because it's the wrong
;; value in the foo fact.  So far, so good.
Jess> (modify (fact-id 1) (me robert))
 <=> f-1 (MAIN::mine (me robert))
 ==> f-4 (MAIN::need-foo (bar bie))
<Fact-1>
Jess> (facts)
f-0   (MAIN::initial-fact)
f-1   (MAIN::mine (me robert))
f-3   (MAIN::foo (bar ber))
f-4   (MAIN::need-foo (bar bie))
For a total of 4 facts.
Jess> (run)
0
Jess>

;; There's a new need-foo fact.  But now, I expected dont-want-two-foos
;; and/or get-a-foo to fire, but neither did.
-----------------------------------

Any ideas why this happened?  I wanted the dont-want-two-foos rule to 
retract the need-foo fact (I used salience earlier, to make sure it 
fired first, but it didn't work then, either).

- Bob

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