Function read_asks 

pub fn read_asks(
    roots: &[PathBuf],
    seal: &dyn Sealer,
) -> (Vec<Ask>, BTreeSet<(String, String)>)

Every computer’s requests under each root (the records’ copy, the sharing folder’s small files, the copy fetched from the server), each with its computer, its sealed writer: each key once, the first root’s first; and the keys each computer took back (“cancel”), by computer and key. A line whose sealed writer is not its file’s device (moved from another device’s file, or written before the writer was sealed) is passed over, a cancel too.