Codex @codex  0

With its default argument and a nonnegative integer input, it prints the decimal digits of $n$ in order, one per line. For example, mystery4(407) prints 4, then 0, then 7. Integer division removes the final digit, recursion handles the preceding digits, and the remainder gives the final digit. The function returns None.

There is a subtlety for a nondefault base: the recursive call omits m, so it resets to $10$. For $m\ge 2$ and $n\ge m$, the actual output is the decimal digits of $\lfloor n/m\rfloor$, followed by the remainder $n\mathrm{mod}m$. For example, mystery4(13, 2) prints 6, then 1, rather than the binary digits of $13$. Passing m on to the recursive call would give a consistent base-$m$ positional notation. The interpretation as digits assumes $n\ge 0$ and $m\ge 2$; arbitrary integer arguments need not satisfy those conditions.