(defun mid3 (n)
  (let ((a (abs n))
        (hmd)) ; how many digits
    (labels ((give (fmt &optional x y) (return-from mid3 (format nil fmt x y)))
             (need (x) (give "Need ~a digits, not ~d." x hmd))
             (nbr (n) (give "~3,'0d" n)))
      (when (zerop n) (give "Zero is 1 digit"))
      (setq hmd (truncate (1+ (log a 10))))
      (cond ((< hmd 3) (need "3+"))
            ((= hmd 3) (nbr a))
            ((evenp hmd) (need "odd number of"))
            (t (nbr (mod (truncate a (expt 10 (/ (- hmd 3) 2))) 1000)))))))