Truth maintenance system issue?

I came across this issue the other day:



Given the code:



(defconcept person (?x))

(defrelation alive (?x))



(defconcept live-person (?x person)

  :<=> (alive ?x)

  )



And the data:

(assert (alive jack))





I can ask :

STELLA> (ask (live-person jack))

TRUE                                                          
;; ok

STELLA> (retract (alive jack)) 

|P?|(ALIVE
JACK)                                        
;; dont know if jack is live now

STELLA> (ask (live-person jack))

UNKNOWN                                                 
;; ok

STELLA> (assert (not (alive jack)))

|P|(NOT (ALIVE
JACK))                                
;; jack is not alive

STELLA> (ask (live-person jack))

FALSE                                                        
;; ok

STELLA> (ask (alive jack))

FALSE                                                        
;; ok

STELLA> (assert (alive jack))

NULL                                                          
;; jack lives again

STELLA> (ask (alive jack))

UNKNOWN                                                 
;; error: i just asserted this. hope I dont need to retract the (not
(alive jack) before asserting the opposite?



STELLA> (ask (live-person jack))

UNKNOWN                                                 
;; error; same as above



On a separate sequence (with different code) I got this log:

....

STELLA> (ask (live-person jack))

UNKNOWN

STELLA> (assert (not (alive jack)))

|P#|(ALIVE JACK)

STELLA> (assert (alive jack))

|P#|(ALIVE JACK)                   Shouldnt
the system warn me of inconsistency when I assert a fact and its
opposite? This is a simple case, but when we have derived facts, this
may be important...

STELLA> (ask (live-person jack))

UNKNOWN                                 
;; why unknown. at least inconsistency should be flagged at query time, if not the time when inconsistent facts were asserted





Thanks

Srini

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