Interestingly, they got one of their own answers wrong.

The question is:

    if a and b are numbers, it is always the case that (a + b) == (b + a)

and their notes on it:

    Is a simple statement involving the commutativity over addition true for floating point? Generally, floating point arithmetic follows the same commutativity laws as real number arithmetic.

They make it clear in their notes that "are numbers" includes infinities but not NaNs. Now consider the case where a = inf and b = -inf. Then inf + (-inf) is NaN, and (-inf) + inf is NaN, and NaN != NaN.

    >>> a = float('inf')
    >>> b = float('-inf')
    >>> a + b == b + a
    False