Function answer 

pub(crate) fn answer(
    qt: &CxxQtThread<Sioul>,
    shared: &Arc<Shared>,
    key: &str,
    answer: &str,
) -> Result<(), String>

Answers an invitation: “accepted”, “tentative” or “declined”. Accepted or maybe, the event goes into your first calendar; then your answer goes to the organizer, from the account the invitation came to. “add” only adds it (an event published, not asked); “remove” takes a cancelled one out.