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Given that equality is not identify, order does matter, although in 3.2.2 the results are the opposite of what one might expect.
a = set((1,2,3))
b = set((1.0, 3.0, 5.0))
print(a&b, b&a)
print(a.intersection(b), b.intersection(a))
a &= b
print(a)
>>> 
{1.0, 3.0} {1, 3}
{1.0, 3.0} {1, 3}
{1.0, 3.0}
In my view, a &= b should remove the members of a that are not in b, rather than deleting all and replacing some with equal members of b.
That glitch aside, & remains and remains binary for exact control of order. The doc should just say that intersection may re-order the intersection for efficiency.