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>>> from functools import partial
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>>> Y = lambda f: partial(f, f)
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>>> fib = lambda f, n: None if n < 0 else (0 if n == 0 else (1 if n == 1 else f(f, n-1) + f(f, n-2)))
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>>> [ Y(fib)(i) for i in range(-2, 10) ]
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[None, None, 0, 1, 1, 2, 3, 5, 8, 13, 21, 34]
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