pub fn send_channel_from<C1, C2, N, A, B>(
    _n: N,
    cont: PartialSession<C2, B>
) -> PartialSession<C1, SendChannel<A, B>> where
    A: Protocol,
    B: Protocol,
    C1: Context,
    C2: Context,
    N: ContextLens<C1, A, Empty, Target = C2>,