(defun radix62 (n)
  (concatenate 'string
			   (nreverse (loop with char = "0123456789abcdefghijklnmopqrstuvwxyzABCDEFGHIJKLNMOPQRSTUVWXYZ"
							   with radix = (length char)
							   for m = n then (floor m radix)
							   until (zerop m)
							   collect (elt char (mod m radix))))))

(let ((n 18446744073709551615))
  (format t "~d==~a~%" n (radix62 n)))