> Instead, CL forces you to do this:

  (if (cond)
      (progn
          (preamble)
          (let ((a 5))
              (do-stuff a)))
      (do-stuff 6))

Above would be unusual. I'd only write code that way if preamble was dependent on some previous binding of a. Note that let subsumes progn in most cases. I'd write it like this:

  (if (cond)
      (let ((a 5))
          (preamble)
          (do-stuff a))
      (do-stuff 6))