Function last_heard 

fn last_heard(entry: Option<&Entry>, memory: &Memory, id: &str) -> i64

When another device was last heard of, from sealed facts only: its entry’s times (sealed, bound to it) and the clock of its last record read here (Memory::heard_at), in milliseconds; 0 when nothing is known. Never its notes in the folder, which are plain and could be forged.