Function import_events 

fn import_events(
    s: &Session,
    plan: &Plan_,
    existing: &[Task],
    dry_run: bool,
    zone: &TimeZone,
) -> Result<BTreeSet<String>, String>

The file’s appointments written: new ones in their calendar, known ones changed line by line, one imported before as a task made the event (the task and its blocks taken out, never both, never neither). The calendars touched, “account/id”.