chain https://docs.python.org/3/library/itertools.html#itertools.chain combinations https://docs.python.org/3/library/itertools.html#itertools.combinations count ...
As of Python 3.13, itertools.tee() now "flattens" other tee objects, as documented in #123885. This behavior change seems to be from Python 3.13 because tee() objects in Python 3.12 and below do not ...