Function format_open 

fn format_open(sharing: &Sharing<'_>, memory: &Memory, now_ms: i64) -> bool

Whether this device may write format 2: another device at least counts (present; alone, it writes format 1: a device joining later may run an older Sioul), and none holds it back (format_holders). A device not joined yet holds nothing format 1 could lose: every other device saying it reads format 2 is enough, and it joins in format 2.