List of known bugs:

* The generic function application MOP is not yet implemented, so it
  is not yet possible to customize the behaviour of GF application by
  specializing apply-generic to a GF class of your own making.

* next-method currently assumes that argument types are the same as
  that for the "parent" method.  Type checking should be introduced
  for the case when the user supplies arguments: (next-method ...)

* Need to check if another thread has already memoized the method
  (Hmm... hasn't this been implemented now? MDJ 000827)
