Support (defun (setf foo) ...)

Camm, I know you’re just about ready to make a new release, but would it 
be possible to add support for setf functions so that thing like |(defun 
(setf foo) ..)| work? I know gcl has a working |defsetf|, but supporting 
these setf defuns would be a nice addition.

But don’t delay the release because of this; it’s a wishlist item that 
would be nice to have but not quite necessary since gcl has |defsetf|.

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