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- http://stackoverflow.com/questions/944700/how-to-check-for-nan-in-python/944712#944712

::

    In [1]: nan = float('nan')

    In [2]: nan is nan
    Out[2]: True

    In [3]: nan == nan
    Out[3]: False

- http://stackoverflow.com/questions/15977808/why-dont-methods-have-reference-equality

- http://stackoverflow.com/questions/3877230/why-does-id-id-and-id-id-in-cpython

- http://stackoverflow.com/questions/306313/pythons-is-operator-behaves-unexpectedly-with-integers