.
        At runtime, functions marked this way should return a boolean.

        ``TypeIs`` aims to benefit *type narrowing* -- a technique used by static
        type checkers to determine a more precise type of an expression within a
        program's code flow.  Usually type narrowing is done by analyzing
        conditional code flow and applying the narrowing to a block of code.  The
        conditional expression here is sometimes referred to as a "type guard".

        Sometimes it would be convenient to use a user-defined boolean function
        as a type guard.  Such a function should use ``TypeIs[...]`` as its
        return type to alert static type checkers to this intention.

        Using  ``-> TypeIs`` tells the static type checker that for a given
        function:

        1. The return value is a boolean.
        2. If the return value is ``True``, the type of its argument
        is the intersection of the type inside ``TypeGuard`` and the argument's
        previously known type.

        For example::

            def is_awaitable(val: object) -> TypeIs[Awaitable[Any]]:
                return hasattr(val, '__await__')

            def f(val: Union[int, Awaitable[int]]) -> int:
                if is_awaitable(val):
                    assert_type(val, Awaitable[int])
                else:
                    assert_type(val, int)

        ``TypeIs`` also works with type variables.  For more information, see
        PEP 742 (Narrowing types with TypeIs).
        c