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I'd like to share a fact that I discovered when working on my
compiler: LET* is not really semantically equivalent to nested LETs,
even though you might think it is (and even though it used to be
implemented as such a macro on the Lisp Machine!).  The reason
is that non-pervasive SPECIAL declarations would have their meanings
altered by such a transformation.  All implementors should understand
this problem to avoid what might otherwise be a tempting but
incorrect implementation.