(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)))
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