Double unquote/unquote-splicing

[Nathan Trapuzzano]

This was a subject of discussion not too long ago.  Someone wrote to say
how unquote and unquote-splicing don't pile up properly (as in ,,@FORM
or ,@,@FORM).  In fact, they do work properly in the case that the inner
unquote-splicing expands to just one form, but this is hardly a useful
case (it's easily re-writable in other terms).  On the other hand, there
are useful cases of ,,@ and ,@,@ that cannot be written without
rendering the code virtually unreadable.  Consider:

(defmacro once-only (names &rest body)
  (let ((gensyms (loop for n in names collect (gensym))))
    `(let (,@(loop for g in gensyms collect `(,g (gensym))))
      `(let (,,@(loop for g in gensyms for n in names collect ``(,,g ,,n)))
        ,(let (,@(loop for n in names for g in gensyms collect `(,n ,g)))
           ,@body)))))

(defmacro once-only (names &rest body)
  (let ((gensyms (loop for n in names collect (gensym))))
    `(let (,@(loop for g in gensyms collect `(,g (gensym))))
       (append (list `let)
               (list (append
                      (list ,@(loop for g in gensyms for n in names
                                    collect `(append (list ,g) (list ,n))))))
               (list
                (let (,@(loop for n in names for g in gensyms
                              collect `(,n ,g)))
                       ,@body))))))

These two macros are semantically equivalent, but one is infinitely more
easy to read thanks to ,,@.  However, only the latter form works in
Emacs when NAMES has length > 1.

For the sake of readability, I'd like to propose properly implementing
,,@ and ,@,@ etc.  Is this doable without major changes to the current
backquote implementation?  If so, is there some other reason why this
should _not_ be done?

Nathan