It's completely undocumented, but today I noticed that Flag.__contains__() is actually a subset operation.


    def __contains__(self, other):
        ...
        return other._value_ & self._value_ == other._value_

It's an unfortunate departure from the `set` type, which uses `in` for membership test and issubset() / `<=` for subset test.

For set operations, the Flag individual bits should be considered the members of a set (not Flag compound values, which are themselves equivalent to a set).