Function cut::cut_append

pub fn cut_append<C1, C2, C3, C4, A, B>(
    cont1: PartialSession<C3, B>, 
    cont2: PartialSession<C2, A>
) -> PartialSession<C4, B> where
    A: Protocol,
    B: Protocol,
    C1: Context + AppendContext<(A, ()), Appended = C3, Appended = C4> + AppendContext<C2>,
    C2: Context,
    C3: Context,
    C4: Context,