Function remove_old_rounds 

fn remove_old_rounds(
    sharing: &Sharing<'_>,
    memory: &Memory,
    readers: &BTreeSet<String>,
)

This computer’s old files, once every device that counts has read past them (readers), and the past rounds of the devices that no longer count by a sealed fact (Memory::departed), here and in the copy fetched from the server beside (remote::overlay): never fetched again, and taken out of the server by the sending (remote).