pub fn unfix_session<N, C1, C2, A, B, R, F>(
    _n: N,
    cont: PartialSession<C2, B>
) -> PartialSession<C1, B> where
    B: Protocol,
    C1: Context,
    C2: Context,
    F: Protocol,
    R: Context,
    F: RecApp<(RecX<R, F>, R), Applied = A>,
    A: Protocol,
    N: ContextLens<C1, RecX<R, F>, A, Target = C2>,