[Barry knows this, but just for completeness...]

Barry Margolin  <barmar@alum.mit.edu> wrote:
+---------------
| The list of special forms in the language spec is the result of 
| experienced language designers and implementors, who know which 
| operators typically need to be recognized as primitives by the 
| implementation.  The spec also says that an implementation is
| allowed to implement any special operator as a macro.
+---------------

And vice-versa [2nd sentence below], with one additional proviso:

    3.1.2.1.2.2 Macro Forms
    ...
    An implementation is free to implement a Common Lisp special
    operator as a macro. An implementation is free to implement
    any macro operator as a special operator, but only if an
    equivalent definition of the macro is also provided.

Presumably this is so that user-written code-walkers that don't
know that some macro is really a special operator in a particular
implementation can still call MACROEXPAND and do (almost) the
right thing.


-Rob

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