The following known problems with existing MOPs are not tested (yet):

- Allegro: defmethod does not accept more than one qualifier.

- SBCL/CMUCL (PCL): The object returned by compute-discriminating-function cannot be a closure. Likewise, the second parameter to set-funcallable-instance-function cannot be a closure, but only a "pure" function/thunk.

- CLisp: A generic function without any method defined on it cannot be called.
