How to do some recursiv inference?

I want to make some recursive inference but I failed...Can you help me?
The definition is like this one :

(deffunction tambah ((?I INTEGER) (?J INTEGER)) :-> (?K INTEGER)
:<<=	(or	(and (= ?I 0) (= ?J ?K))
		(and (dec ?I ?I2) (inc ?J ?J2) (= (tambah ?I2 ?J2) ?K))))
(defrelation inc(?I ?J))
(defrelation dec(?I ?J))
(assert (inc 1 2))
(assert (inc 0 1))
(assert (inc 2 3))
(assert (dec 2 1))
(assert (dec 1 0))
(assert (dec 3 2))

then after I ask

(defrelation rtambah (?I ?J ?K) :<<= (= (tambah ?I ?J) ?K))

I got these messages.....

Warning: Derived both TRUE and FALSE for the proposition `FALSE'
   Clash occurred in the world `|WLD|52' in module 
`|MDL|/PL-KERNEL-KB/PL-USER'.

Tried to bind `|V|?x3' to NULL value.  Last time we saw
   this bug, it was caused by COLLECT-INTO-LIST applied to
   KAPPA with unbound external variables.
()

What is wrong with my definition.

PS: for your information, i still use PowerLoom 2.0
Danke!

--
Mohammad Ridwan Agustiawan ::: 13599066
Laboratorium Grafika dan Intelijensia Buatan
Departemen Teknik Informatika Fakultas Teknologi Industri
Institut Teknologi Bandung
mail:: [email protected]
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