Solution to the 99 LISP Problems #16

This seemingly innocuous problem turned out to be a little challenging too. Like I have mentioned earlier, I strongly prefer a pure functional or recursive solution. Therefore, carrying the state around (in this case, the position of the current in the list) is slightly tough. I knew I should be using closures. I have used some sort of let + lambda. Not sure if this is elegant.

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(defun drop (alist n)
  (let ((i 1))
    (mapcan
      (lambda (x)
        (let ((j i))
          (setf i (1+ i))
          (if (= (mod j n) 0) nil (list x))))
      alist)))

Lisp dialect: Steel Bank Common Lisp