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The operator -= modifies a frozenset (this should not be possible),
instead of signaling a TypeError. Contrast with the += operator.
>>> f=frozenset([1,2])
>>> f
frozenset([1, 2])
>>> f -= frozenset([1])
>>> f
frozenset([2])
>>> f -= frozenset([2])
>>> f
frozenset([])
>>> f += frozenset([2])
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +=: 'frozenset' and 'frozenset'
>>>