(let* ((char "0123456789abcdefghijklnmopqrstuvwxyzABCDEFGHIJKLNMOPQRSTUVWXYZ")
       (radix (length char)))
  (defun radix62 (n &optional (ret nil))
    (if (zerop n)
		(concatenate 'string ret)
		(radix62 (floor n radix) (cons (elt char (mod n radix)) ret)))))

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