pub fn unwrap_session<N, C, T, A>(
    N,
    cont: PartialSession<<N as ContextLens<C, Wrap<T>, <T as Wrapper>::Unwrap>>::Target, A>
) -> PartialSession<C, A> where
    C: Context,
    A: Protocol,
    T: 'static + Wrapper + Send,
    N: ContextLens<C, Wrap<T>, <T as Wrapper>::Unwrap>,