Function act_many 

pub(crate) fn act_many(
    qt: &CxxQtThread<Sioul>,
    shared: &Arc<Shared>,
    keys: &[String],
    action: &str,
)

The same act on several messages (a selection, the review queue’s “for all”): reading and flagging at once, the rest under one “Undo”; “spam” and “not-spam” as each message’s folder asks (act).