Then I thought "this is stupid!  Python already does reference
counting."  Voila!  Vast swaths of tedious code vanished, giving this
remarkably simple implementation:

    def mytee(xs, n):
        last = [None, None]

        def gen(it, mylast):
            nonlocal last
            while True:
                mylast = mylast[1]
                if not mylast:
                    mylast = last[1] = last = [next(it), None]
                yield mylast[0]

        it = iter(xs)
        return tuple(gen(it, last) for _ in range(n))

There's no need to keep a pointer to the start of the shared list at
all - we only need a pointer to the end of the list ("last"), and each
derived generator only needs a pointer to its own current position in
the list ("mylast").

What I find kind of hilarious is that it's no help at all as a
prototype for a C implementation:  Python recycles stale `[next(it),
None]` pairs all by itself, when their internal refcounts fall to 0.
That's the hardest part.

BTW, I certainly don't suggest adding this to the itertools docs
either.  While it's short and elegant, it's too subtle to grasp easily
- if you think "it's obvious", you haven't yet thought hard enough
about the problem ;-)
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