problem with defstruct

[Bjoern Hoefling <hoefling@dfki.uni-kl.de>]

A friend of me has a problem with clisp and asked me to post it in the clisp
mailing list. Here is a short script:
________________________________________________________________________________
> (defstruct (object             
            (:conc-name object-)
            (:predicate object?)
           )
    (reference    'NUNA)
    )
OBJECT
> (setq a (make-object))
#S(OBJECT :REFERENCE NUNA)
> (object-reference a)
NUNA
> (defstruct (unknown (:predicate unknown?)
                    (:conc-name object-)
                    (:include object)
                    )
  (abstract-functions   nil :type list)
)
UNKNOWN
> (object-reference a)

*** - SYSTEM::%STRUCTURE-REF: #S(OBJECT :REFERENCE NUNA) ist keine Structure vom Typ UNKNOWN.
1. Break> 
________________________________________________________________________________

Thanks for any help
Bjoern Hoefling (hoefling@dfki.uni-kl.de)