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nested backquotes

I think I understand your description (I think you left out a quote or
two in the last element).  Furthermore, I strongly suspect (but have
not yet proven) that it is equivalent to mine.  If so, my explanation
seems easier to understand.  The only problem was that you evidently
did not understand what "matching" meant.  It was very simple and did
NOT require expanding the inner backquotes.  Let me rephrase the model
and see if you agree that it gives the same results as yours.  

Pretend that comma (comma@) reads like quote - ,x = (COMMA x) except
that an expression may not be in the scope of more commas than backquotes.
Similarly, suppose that backquote reads as the symbol BACKQUOTE consed
onto the next read.  So ``(a ,s ,',d ,,f `(,g)) reads as
Now in order to evaluate that expression we find all forms that are
in the scope of as many COMMAs as BACKQUOTEs.  We discard the leading
BACKQUOTE and replace such expressions (along with the matching COMMAs)
with their values.  In this case, the d and f are in the scope of 2 of 
each.  The result is
Evaluating this would similarly give something like
(a 3 4 7 (BACKQUOTE ((COMMA g))))

If you don't believe this model is equivalent please send me a 
counterexample.  (The only thing I can see won't work is if something
EVALUATES to a list containing the symbols BACKQUOTE or COMMA, and I
think we can pretend that's impossible, even though it's precisely what
I really WANTED to do!)