Function invitation_answer 

pub fn invitation_answer(
    from: (&str, &str),
    to: &str,
    subject: &str,
    text: &str,
    ics: &str,
    date: i64,
) -> Result<Outgoing, String>

An answer to an invitation (iMIP, RFC 6047 §2.4): a sentence for people, and the iTIP REPLY for their calendar, as two versions of one message.