Error messages on simple backward chainer variation

[ngeiswei]

What is your problem?

MeTTaLog generates tons of error messages on a simple backward chainer variation.

How to reproduce your problem?

Run the following metta code

;; Knowledge base
!(bind! &kb (new-space))
!(add-atom &kb (: AND (-> Boolean Boolean Boolean)))
!(add-atom &kb (: X Boolean))

;; Backward chainer
;; Base case
(= (bc $kb $_ (: $prg $type)) (match $kb (: $prg $type) (: $prg $type)))
;; Recursive steps
;; Unary operators
(= (bc $kb (S $k) (: ($fun $arg) $type))
   (let* (((: $fun (-> $prms $type)) (bc $kb $k (: $fun (-> $prms $type))))
          ((: $arg $prms) (bc $kb $k (: $arg $prms))))
     (: ($fun $arg) $type)))
;; Binary operators
(= (bc $kb (S $k) (: ($fun $arg1 $arg2) $type))
   (let* (((: $fun (-> $prms1 $prms2 $type))
           (bc $kb $k (: $fun (-> $prms1 $prms2 $type))))
          ((: $arg1 $prms1) (bc $kb $k (: $arg1 $prms1)))
          ((: $arg2 $prms2) (bc $kb $k (: $arg2 $prms2))))
     (: ($fun $arg1 $arg2) $type)))

;; Test bc
!(bc &kb (S (S Z)) (: $prg Boolean))

What do you get?

At some point the MeTTaLog interpreter outputs

#(failed #( : user once ) #( : user #(if_or_else #(eval = $10000 494 &self (return X) $10001) #(call eval (return X) $10001)) ))


#(on_mettalog_error #(failed #( : user once ) #( : user #(if_or_else #(eval = $10000 494 &self (return X) $10001) #(call eval (return X) $10001)) )))

^  Call: (283) [user] call(user:once, user:if_or_else(eval(=, _954712, 494, '&self', [return, 'X'], _955146), call(eval, [return, 'X'], _955146)))
...

What should you expect instead?

[(: X Boolean), (: (AND X X) Boolean), (: (AND X (AND X X)) Boolean), (: (AND (AND X X) X) Boolean), (: (AND (AND X X) (AND X X)) Boolean)]

What else do you have to say?

Tested on revision 5241cf1.