Function replacements_hold 

pub fn replacements_hold(
    memory: &Path,
) -> impl Fn() -> Option<Box<dyn Held>> + '_

The exchange lock as sioul_core::draftcopies::Hold asks it, for the copies beside drafts encrypted again: taken for each short batch of their replacements, waiting for an exchange running, so that no exchange reads a copy while it is replaced (nor seals and sends the zeros written over the old one after). A device that has never shared (no share/ folder beside memory) runs no exchange: a guard that holds nothing, no folder made.