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When I thought about the same problem for PyPy, I imagined
that it would be easy to use the call graph computed by
the type inferencer ("annotator").  We would find an
algorithm that figures out the minimal number of places
that need a Py_EnterRecursiveCall so that every cycle goes
through at least one of them.  For CPython it might be
possible to go down the same path if someone can find a C
code analyzer smart enough to provide the required
information -- a call graph including indirect calls through
function pointers.  Not sure it's sane, though.