I agree. I think `rethorws` for `beginAsync` is problematic. For example, what happens when the `foo` in the following code throws an `Error` asynchronously? func foo() async throws { ... } beginAsync(foo) `foo` is acceptable as `beginAsync`'s `body` by its type. However its error might be thrown asynchronously and it is impossible to rethrow it. So the error must be ignored or treated as an universal error by untyped propagation. It breaks type safety about error handling. So I think the signature of `beginAsync` should be the following one. func beginAsync(_ body: () async -> Void) -> Void
