pub fn send_channel_to<N, M, C1, C2, C3, A1, A2, B>(
    _n: N,
    _m: M,
    cont: PartialSession<C3, B>
) -> PartialSession<C1, B> where
    C1: Context,
    C2: Context,
    C3: Context,
    A1: Protocol,
    A2: Protocol,
    B: Protocol,
    N: ContextLens<C2, ReceiveChannel<A1, A2>, A2, Target = C3>,
    M: ContextLens<C1, A1, Empty, Target = C2>,