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subtypep on structures

According to my reading of the common lisp manual (and corroborated by
Allegro CL), defstruct types formed with :INCLUDEs should be subtypes of
the :INCLUDEd structure.  So, after defining

  (defstruct foo)
  (defstruct (bar (:include foo)))

the expression

  (subtypep 'bar 'foo)

should return T T, not NIL NIL as it does.  Note that 
  (foo-p (make-bar))

properly returns T.

This bug really matters when trying to use structure types for
presentation objects.  Any ideas or workarounds?

--Mike Wellman.