Why doesn't in operator type guard work with Sequence ?

[robbi]

As mentioned in https://github.com/microsoft/pyright/blob/main/docs/type-concepts-advanced.md#type-guards type guards in and not in works for list, set, frozenset, deque, tuple, dict, defaultdict, and OrderedDict.
Why is Sequence not supported ?

[erictraut]

I don't think Sequence guarantees the semantics for in that are required for safe type narrowing. The other classes in this list have well-documented and guaranteed behaviors in this regard.