Function fill_heard_at 

fn fill_heard_at(
    folder: &Path,
    key: &[u8; 32],
    computer: &str,
    memory: &mut Memory,
    now_ms: i64,
)

The clock of each other device’s last record in the folder, opened (sealed, bound to its device, round, number and clock), for the devices this memory has not heard of yet (Memory::heard_at): a memory from before it was kept learns when each device it read last wrote.

A device read here whose rounds are gone from the folder takes the latest clock of its changes this memory still holds (each the clock of a record read here, sealed); one with none at all is noted as found without a clock (Memory::unclocked), at now_ms, once.