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This doesn't modify f, it replaces it with a new frozenset:
 >>> f = frozenset({1, 2})
 >>> f
 frozenset({1, 2})
 >>> id(f)
 3071990668
 >>> f -= frozenset({1})
 >>> f
 frozenset({2})
 >>> id(f)
 3066719340
Notice how the two ids are different.
In other words,
 f -= frozenset({1})
is equivalent to
 f = f - frozenset({1})
You get an error with += because
 f = f + frozenset({1})
is not a valid operation for (frozen)sets.